diff --git a/Sources/StreamRecorderKit/FFmpegRecorder.swift b/Sources/StreamRecorderKit/FFmpegRecorder.swift
index 58c7c6f..c3566fb 100644
--- a/Sources/StreamRecorderKit/FFmpegRecorder.swift
+++ b/Sources/StreamRecorderKit/FFmpegRecorder.swift
@@ -139,7 +139,9 @@ public final class FFmpegRecorder: StreamRecorderEngine {
         }
         
         guard FileManager.default.fileExists(atPath: outputPath) else {
-            throw RecordingError.outputNotFound
+            let lastStderr = String(stderrAccumulator.suffix(2000))
+            NSLog("[Recorder] ffmpeg stderr (last 2000 chars):\n%@", lastStderr)
+            throw RecordingError.outputNotFound(lastStderr)
         }
         
         // 如果是手动停止，跳过耗时的分析（fragmented MP4 可能导致 ffprobe 卡住）
diff --git a/Sources/StreamRecorderKit/StreamRecorderEngine.swift b/Sources/StreamRecorderKit/StreamRecorderEngine.swift
index d921ffa..5678cd4 100644
--- a/Sources/StreamRecorderKit/StreamRecorderEngine.swift
+++ b/Sources/StreamRecorderKit/StreamRecorderEngine.swift
@@ -58,7 +58,7 @@ public enum RecordingError: Error, LocalizedError {
     case noFFmpeg
     case noMediaTracks
     case processFailed(Int, String? = nil)
-    case outputNotFound
+    case outputNotFound(String? = nil)
     case unknown(String)
     
     public var errorDescription: String? {
@@ -72,7 +72,12 @@ public enum RecordingError: Error, LocalizedError {
                 return "录制失败 (exit code: \(code)):\n\(preview)"
             }
             return "录制失败 (exit code: \(code))"
-        case .outputNotFound: return "输出文件不存在"
+        case .outputNotFound(let stderr):
+            if let s = stderr, !s.isEmpty {
+                let preview = String(s.prefix(300))
+                return "输出文件不存在\n\(preview)"
+            }
+            return "输出文件不存在"
         case .unknown(let msg): return msg
         }
     }