diff --git a/frontend/src/pages/HomePage.vue b/frontend/src/pages/HomePage.vue
index 7688a67..f1e47bc 100644
--- a/frontend/src/pages/HomePage.vue
+++ b/frontend/src/pages/HomePage.vue
@@ -20,8 +20,13 @@ interface UploadItem {
 
 const auth = useAuthStore()
 
+type CompressionMode = 'percent' | 'size'
+
 const options = reactive({
+  mode: 'percent' as CompressionMode,  // 压缩模式：百分比 / 目标大小
   compressionRate: 60,
+  targetSize: '' as string,  // 目标大小数值
+  targetUnit: 'KB' as 'KB' | 'MB',  // 目标大小单位
   maxWidth: '' as string,
   maxHeight: '' as string,
 })
@@ -105,18 +110,33 @@ function toInt(v: string): number | undefined {
   return Math.floor(n)
 }
 
+function getTargetSizeBytes(): number | undefined {
+  if (options.mode !== 'size') return undefined
+  const size = Number(options.targetSize)
+  if (!Number.isFinite(size) || size <= 0) return undefined
+  return options.targetUnit === 'MB' ? size * 1024 * 1024 : size * 1024
+}
+
 async function runOne(item: UploadItem) {
   item.status = 'compressing'
   item.error = undefined
 
   try {
+    const compressOptions = options.mode === 'percent'
+      ? {
+          compression_rate: options.compressionRate,
+          max_width: toInt(options.maxWidth),
+          max_height: toInt(options.maxHeight),
+        }
+      : {
+          target_size_bytes: getTargetSizeBytes(),
+          max_width: toInt(options.maxWidth),
+          max_height: toInt(options.maxHeight),
+        }
+
     const result = await compressFile(
       item.file,
-      {
-        compression_rate: options.compressionRate,
-        max_width: toInt(options.maxWidth),
-        max_height: toInt(options.maxHeight),
-      },
+      compressOptions,
       auth.token,
     )
 
@@ -460,7 +480,31 @@ async function resendVerification() {
           <div class="text-sm font-medium text-slate-900">压缩参数</div>
 
           <div class="mt-4 grid grid-cols-1 gap-4">
-            <label class="space-y-1">
+            <!-- 压缩模式切换 -->
+            <div class="space-y-2">
+              <div class="text-xs font-medium text-slate-600">压缩模式</div>
+              <div class="flex rounded-lg border border-slate-200 p-1">
+                <button
+                  type="button"
+                  class="flex-1 rounded-md px-3 py-1.5 text-xs font-medium transition"
+                  :class="options.mode === 'percent' ? 'bg-indigo-600 text-white' : 'text-slate-600 hover:bg-slate-100'"
+                  @click="options.mode = 'percent'"
+                >
+                  按百分比
+                </button>
+                <button
+                  type="button"
+                  class="flex-1 rounded-md px-3 py-1.5 text-xs font-medium transition"
+                  :class="options.mode === 'size' ? 'bg-indigo-600 text-white' : 'text-slate-600 hover:bg-slate-100'"
+                  @click="options.mode = 'size'"
+                >
+                  按目标大小
+                </button>
+              </div>
+            </div>
+
+            <!-- 百分比模式 -->
+            <label v-if="options.mode === 'percent'" class="space-y-1">
               <div class="flex items-center justify-between text-xs font-medium text-slate-600">
                 <span>压缩率</span>
                 <span class="text-slate-500">{{ options.compressionRate }}%</span>
@@ -476,6 +520,28 @@ async function resendVerification() {
               <div class="text-xs text-slate-500">数值越小压缩越强，目标为压缩后体积占原图比例（100% 为不压缩）。</div>
             </label>
 
+            <!-- 目标大小模式 -->
+            <div v-else class="space-y-1">
+              <div class="text-xs font-medium text-slate-600">目标大小</div>
+              <div class="flex gap-2">
+                <input
+                  v-model="options.targetSize"
+                  type="number"
+                  min="1"
+                  placeholder="例如 50"
+                  class="flex-1 rounded-md border border-slate-200 bg-white px-3 py-2 text-sm text-slate-800"
+                />
+                <select
+                  v-model="options.targetUnit"
+                  class="rounded-md border border-slate-200 bg-white px-3 py-2 text-sm text-slate-800"
+                >
+                  <option value="KB">KB</option>
+                  <option value="MB">MB</option>
+                </select>
+              </div>
+              <div class="text-xs text-slate-500">直接指定压缩后的目标大小，系统会自动调整质量以逼近目标（仅 JPEG/WebP/AVIF 支持）。</div>
+            </div>
+
             <div class="grid grid-cols-2 gap-3">
               <label class="space-y-1">
                 <div class="text-xs font-medium text-slate-600">最大宽度（px）</div>
diff --git a/frontend/src/services/api.ts b/frontend/src/services/api.ts
index f78e1a5..0de5731 100644
--- a/frontend/src/services/api.ts
+++ b/frontend/src/services/api.ts
@@ -84,6 +84,7 @@ export interface CompressResponse {
 export interface CompressOptions {
   level?: CompressionLevel
   compression_rate?: number
+  target_size_bytes?: number  // 新增：直接指定目标大小（字节）
   output_format?: OutputFormat
   max_width?: number
   max_height?: number
@@ -96,6 +97,7 @@ export async function compressFile(file: File, options: CompressOptions, token?:
 
   if (options.level) form.append('level', options.level)
   if (options.compression_rate) form.append('compression_rate', String(options.compression_rate))
+  if (options.target_size_bytes) form.append('target_size_bytes', String(options.target_size_bytes))
   if (options.output_format) form.append('output_format', options.output_format)
   if (options.max_width) form.append('max_width', String(options.max_width))
   if (options.max_height) form.append('max_height', String(options.max_height))
diff --git a/src/api/compress.rs b/src/api/compress.rs
index 67c1466..24c3ded 100644
--- a/src/api/compress.rs
+++ b/src/api/compress.rs
@@ -54,6 +54,7 @@ struct CompressRequest {
     file_bytes: Vec<u8>,
     level: CompressionLevel,
     compression_rate: Option<u8>,
+    target_size_bytes: Option<u64>,  // 新增：直接指定目标大小（字节）
     output_format: Option<ImageFmt>,
     max_width: Option<u32>,
     max_height: Option<u32>,
@@ -246,6 +247,7 @@ async fn compress_json(
             format_out,
             effective_level,
             req.compression_rate,
+            req.target_size_bytes,  // 新增：目标大小
             req.max_width,
             req.max_height,
             req.preserve_metadata,
@@ -603,6 +605,7 @@ async fn compress_direct(
             format_out,
             effective_level,
             req.compression_rate,
+            req.target_size_bytes,  // 新增：目标大小
             req.max_width,
             req.max_height,
             req.preserve_metadata,
@@ -835,6 +838,7 @@ async fn parse_single_file_request(multipart: &mut Multipart) -> Result<Compress
     let mut level = CompressionLevel::Medium;
     let mut output_format: Option<ImageFmt> = None;
     let mut compression_rate: Option<u8> = None;
+    let mut target_size_bytes: Option<u64> = None;  // 新增
     let mut max_width: Option<u32> = None;
     let mut max_height: Option<u32> = None;
     let mut preserve_metadata = false;
@@ -905,6 +909,24 @@ async fn parse_single_file_request(multipart: &mut Multipart) -> Result<Compress
                     "1" | "true" | "yes" | "y" | "on"
                 );
             }
+            "target_size_bytes" | "target_size" => {
+                let v = text.trim();
+                if !v.is_empty() {
+                    target_size_bytes = Some(
+                        v.parse::<u64>()
+                            .map_err(|_| AppError::new(ErrorCode::InvalidRequest, "target_size_bytes 格式错误，需为正整数（字节）"))?,
+                    );
+                    // 最小目标大小限制：1KB
+                    if let Some(size) = target_size_bytes {
+                        if size < 1024 {
+                            return Err(AppError::new(
+                                ErrorCode::InvalidRequest,
+                                "target_size_bytes 最小为 1024（1KB）",
+                            ));
+                        }
+                    }
+                }
+            }
             _ => {}
         }
     }
@@ -917,6 +939,7 @@ async fn parse_single_file_request(multipart: &mut Multipart) -> Result<Compress
         file_bytes,
         level,
         compression_rate,
+        target_size_bytes,  // 新增
         output_format,
         max_width,
         max_height,
diff --git a/src/services/compress.rs b/src/services/compress.rs
index acd319c..366965f 100644
--- a/src/services/compress.rs
+++ b/src/services/compress.rs
@@ -188,6 +188,7 @@ pub async fn compress_image_bytes(
     format_out: ImageFmt,
     level: CompressionLevel,
     compression_rate: Option<u8>,
+    target_size_bytes: Option<u64>,  // 新增：直接指定目标大小（字节）
     max_width: Option<u32>,
     max_height: Option<u32>,
     preserve_metadata: bool,
@@ -203,7 +204,11 @@ pub async fn compress_image_bytes(
     }
 
     let retention_rate = effective_rate(compression_rate, level);
-    let target_size = compression_rate.map(|value| target_size_from_rate(original_size, value));
+    // 优先使用直接指定的目标大小，其次根据百分比计算
+    let target_size = match target_size_bytes {
+        Some(bytes) => Some(bytes),
+        None => compression_rate.map(|value| target_size_from_rate(original_size, value)),
+    };
 
     if compression_rate == Some(100)
         && format_in == format_out
@@ -439,24 +444,157 @@ fn encode_avif_raw(raw: &[u8], w: u32, h: u32, quality: u8) -> Result<Vec<u8>, A
 }
 
 fn encode_jpeg_target(image: DynamicImage, target_size: u64) -> Result<Vec<u8>, AppError> {
-    let rgb = image.to_rgb8();
-    let (w, h) = rgb.dimensions();
-    let raw = rgb.into_raw();
-    encode_target_quality(35, 95, target_size, |q| encode_jpeg_raw(&raw, w, h, q))
+    encode_with_auto_resize(image, target_size, 1, 95, |img, q| {
+        let rgb = img.to_rgb8();
+        let (w, h) = rgb.dimensions();
+        encode_jpeg_raw(rgb.as_raw(), w, h, q)
+    })
 }
 
 fn encode_webp_target(image: DynamicImage, target_size: u64) -> Result<Vec<u8>, AppError> {
-    let rgba = image.to_rgba8();
-    let (w, h) = rgba.dimensions();
-    let raw = rgba.into_raw();
-    encode_target_quality(30, 95, target_size, |q| encode_webp_raw(&raw, w, h, q))
+    encode_with_auto_resize(image, target_size, 1, 95, |img, q| {
+        let rgba = img.to_rgba8();
+        let (w, h) = rgba.dimensions();
+        encode_webp_raw(rgba.as_raw(), w, h, q)
+    })
 }
 
 fn encode_avif_target(image: DynamicImage, target_size: u64) -> Result<Vec<u8>, AppError> {
-    let rgba = image.to_rgba8();
-    let (w, h) = rgba.dimensions();
-    let raw = rgba.into_raw();
-    encode_target_quality(35, 90, target_size, |q| encode_avif_raw(&raw, w, h, q))
+    encode_with_auto_resize(image, target_size, 1, 95, |img, q| {
+        let rgba = img.to_rgba8();
+        let (w, h) = rgba.dimensions();
+        encode_avif_raw(rgba.as_raw(), w, h, q)
+    })
+}
+
+/// 支持自动缩放尺寸的目标大小压缩
+/// 当仅调整质量无法达到目标大小时，自动缩小图片尺寸
+fn encode_with_auto_resize<F>(
+    image: DynamicImage,
+    target_size: u64,
+    min_q: u8,
+    max_q: u8,
+    mut encode_fn: F,
+) -> Result<Vec<u8>, AppError>
+where
+    F: FnMut(&DynamicImage, u8) -> Result<Vec<u8>, AppError>,
+{
+    let (orig_w, orig_h) = image.dimensions();
+    let min_dimension = 16u32; // 最小尺寸限制
+
+    // 首先尝试用最低质量压缩原始尺寸
+    let min_q_result = encode_fn(&image, min_q)?;
+    if min_q_result.len() as u64 <= target_size {
+        // 最低质量已满足，用二分法找最佳质量
+        return encode_target_quality_with_image(&image, min_q, max_q, target_size, &mut encode_fn);
+    }
+
+    // 需要缩放：根据当前大小和目标大小计算缩放比例
+    let current_size = min_q_result.len() as u64;
+    // 文件大小大致与像素数成正比，所以尺寸缩放系数 = sqrt(目标大小/当前大小)
+    let scale = ((target_size as f64 / current_size as f64).sqrt() * 0.9).min(1.0); // 0.9 为安全系数
+
+    let mut best_result = min_q_result;
+    let mut best_is_under = false;
+
+    // 尝试多个缩放级别
+    let scales = [scale, scale * 0.8, scale * 0.6, scale * 0.4, 0.3, 0.2, 0.1];
+
+    for &s in &scales {
+        let new_w = ((orig_w as f64 * s).round() as u32).max(min_dimension);
+        let new_h = ((orig_h as f64 * s).round() as u32).max(min_dimension);
+
+        if new_w < min_dimension && new_h < min_dimension {
+            break; // 达到最小尺寸
+        }
+
+        let resized = image.resize(new_w, new_h, image::imageops::FilterType::Lanczos3);
+
+        // 对缩放后的图片进行二分质量搜索
+        let result = encode_target_quality_with_image(&resized, min_q, max_q, target_size, &mut encode_fn)?;
+        let result_size = result.len() as u64;
+
+        if result_size <= target_size {
+            // 找到满足条件的结果
+            if !best_is_under || result_size > best_result.len() as u64 {
+                // 优先选择更大的（更接近目标且不超过）
+                best_result = result;
+                best_is_under = true;
+            }
+            break; // 已找到满足条件的最大尺寸
+        } else if !best_is_under && result_size < best_result.len() as u64 {
+            // 还没找到满足条件的，保存最接近的
+            best_result = result;
+        }
+    }
+
+    Ok(best_result)
+}
+
+/// 对给定图片进行二分质量搜索
+fn encode_target_quality_with_image<F>(
+    image: &DynamicImage,
+    min_q: u8,
+    max_q: u8,
+    target_size: u64,
+    encode_fn: &mut F,
+) -> Result<Vec<u8>, AppError>
+where
+    F: FnMut(&DynamicImage, u8) -> Result<Vec<u8>, AppError>,
+{
+    let mut best: Option<Vec<u8>> = None;
+    let mut best_diff = u64::MAX;
+    let mut best_is_under = false;
+    let mut best_size = 0u64;
+
+    let mut consider = |bytes: Vec<u8>| {
+        let size = bytes.len() as u64;
+        let is_under = size <= target_size;
+        let diff = if size > target_size {
+            size - target_size
+        } else {
+            target_size - size
+        };
+
+        let should_update = match (best_is_under, is_under) {
+            (false, true) => true,
+            (true, false) => false,
+            _ => diff < best_diff,
+        };
+
+        if should_update {
+            best_diff = diff;
+            best_is_under = is_under;
+            best_size = size;
+            best = Some(bytes);
+        }
+    };
+
+    // 先尝试两端
+    consider(encode_fn(image, min_q)?);
+    if min_q != max_q {
+        consider(encode_fn(image, max_q)?);
+    }
+
+    // 二分查找
+    let mut low = min_q;
+    let mut high = max_q;
+    for _ in 0..10 {
+        if low > high {
+            break;
+        }
+        let mid = (low + high) / 2;
+        let bytes = encode_fn(image, mid)?;
+        let size = bytes.len() as u64;
+        consider(bytes);
+        if size > target_size {
+            high = mid.saturating_sub(1);
+        } else {
+            low = mid.saturating_add(1);
+        }
+    }
+
+    best.ok_or_else(|| AppError::new(ErrorCode::CompressionFailed, "压缩失败"))
 }
 
 fn encode_target_quality<F>(
@@ -470,35 +608,53 @@ where
 {
     let mut best: Option<Vec<u8>> = None;
     let mut best_diff = u64::MAX;
+    let mut best_is_under = false;  // 记录最佳结果是否小于目标
+    let mut best_size = 0u64;
 
-    let mut consider = |bytes: Vec<u8>| {
+    // 考虑一个候选结果
+    let mut consider = |bytes: Vec<u8>, best: &mut Option<Vec<u8>>, best_diff: &mut u64, best_is_under: &mut bool, best_size: &mut u64| {
         let size = bytes.len() as u64;
+        let is_under = size <= target_size;
         let diff = if size > target_size {
             size - target_size
         } else {
             target_size - size
         };
-        if diff < best_diff {
-            best_diff = diff;
-            best = Some(bytes);
+
+        // 优先选择不超过目标大小的结果
+        let should_update = match (*best_is_under, is_under) {
+            (false, true) => true,  // 当前小于目标，之前大于目标 -> 更新
+            (true, false) => false, // 当前大于目标，之前小于目标 -> 不更新
+            _ => diff < *best_diff,  // 同类情况，选择更接近的
+        };
+
+        if should_update {
+            *best_diff = diff;
+            *best_is_under = is_under;
+            *best_size = size;
+            *best = Some(bytes);
         }
     };
 
-    consider(encode(min_q)?);
+    // 先尝试两端
+    let bytes = encode(min_q)?;
+    consider(bytes, &mut best, &mut best_diff, &mut best_is_under, &mut best_size);
     if min_q != max_q {
-        consider(encode(max_q)?);
+        let bytes = encode(max_q)?;
+        consider(bytes, &mut best, &mut best_diff, &mut best_is_under, &mut best_size);
     }
 
+    // 二分查找，增加迭代次数到 12 次以提高精度
     let mut low = min_q;
     let mut high = max_q;
-    for _ in 0..7 {
+    for _ in 0..12 {
         if low > high {
             break;
         }
         let mid = (low + high) / 2;
         let bytes = encode(mid)?;
         let size = bytes.len() as u64;
-        consider(bytes);
+        consider(bytes, &mut best, &mut best_diff, &mut best_is_under, &mut best_size);
         if size > target_size {
             high = mid.saturating_sub(1);
         } else {
@@ -506,6 +662,19 @@ where
         }
     }
 
+    // 精细调整：如果当前结果超出目标太多，尝试更低质量
+    if best_size > target_size {
+        let mut q = min_q;
+        while q <= min_q.saturating_add(5) && q <= max_q {
+            let bytes = encode(q)?;
+            consider(bytes, &mut best, &mut best_diff, &mut best_is_under, &mut best_size);
+            if best_size <= target_size {
+                break;  // 已找到满足条件的结果
+            }
+            q = q.saturating_add(1);
+        }
+    }
+
     best.ok_or_else(|| AppError::new(ErrorCode::CompressionFailed, "压缩失败"))
 }
 
diff --git a/src/worker/mod.rs b/src/worker/mod.rs
index 3aaf3f6..8ff0602 100644
--- a/src/worker/mod.rs
+++ b/src/worker/mod.rs
@@ -384,6 +384,7 @@ async fn process_task_file(
         format_out,
         level,
         compression_rate,
+        None,  // target_size_bytes: worker 批量任务不支持精确大小
         max_width,
         max_height,
         ctx.preserve_metadata,