废话不多说，直接上转换代码

+ (NSURL *)jjMovConvert2Mp4:(NSURL *)movUrl
{
    NSURL *mp4Url = nil;
    AVURLAsset *avAsset = [AVURLAsset URLAssetWithURL:movUrl options:nil];
    NSArray *compatiblePresets = [AVAssetExportSession exportPresetsCompatibleWithAsset:avAsset];
    
    if ([compatiblePresets containsObject:AVAssetExportPresetHighestQuality]) {
        AVAssetExportSession *exportSession = [[AVAssetExportSession alloc]initWithAsset:avAsset
                                                                              presetName:AVAssetExportPresetHighestQuality];
        NSString *mp4Path = [NSString stringWithFormat:@"%@/%d%d.mp4", [self dataPath], (int)[[NSDate date] timeIntervalSince1970], arc4random() % 100000];
        mp4Url = [NSURL fileURLWithPath:mp4Path];
        exportSession.outputURL = mp4Url;
        exportSession.shouldOptimizeForNetworkUse = YES;
        exportSession.outputFileType = AVFileTypeMPEG4;
        dispatch_semaphore_t wait = dispatch_semaphore_create(0l);
        [exportSession exportAsynchronouslyWithCompletionHandler:^{
            switch ([exportSession status]) {
                case AVAssetExportSessionStatusFailed: {
                    NSLog(@"failed, error:%@.", exportSession.error);
                } break;
                case AVAssetExportSessionStatusCancelled: {
                    NSLog(@"cancelled.");
                } break;
                case AVAssetExportSessionStatusCompleted: {
                    NSLog(@"completed.");
                } break;
                default: {
                    NSLog(@"others.");
                } break;
            }
            dispatch_semaphore_signal(wait);
        }];
        long timeout = dispatch_semaphore_wait(wait, DISPATCH_TIME_FOREVER);
        if (timeout) {
            NSLog(@"timeout.");
        }
        if (wait) {
            //dispatch_release(wait);
            wait = nil;
        }
    }
    
    return mp4Url;
}

+ (NSString*)dataPath
{
    NSString *dataPath = [NSString stringWithFormat:@"%@/Library/appdata/chatbuffer", NSHomeDirectory()];
    NSFileManager *fm = [NSFileManager defaultManager];
    if(![fm fileExistsAtPath:dataPath]){
        [fm createDirectoryAtPath:dataPath
      withIntermediateDirectories:YES
                       attributes:nil
                            error:nil];
    }
    return dataPath;
}

比如你是通过UIImagePickerController选择的视频文件，
在代理方法回调中：

- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary<UIImagePickerControllerInfoKey,id> *)info {
  NSURL *url = info[@"UIImagePickerControllerMediaURL"];
            //调用MOV转换成Mp4
            NSString *filePath = [[NSURL  jjMovConvert2Mp4:url] absoluteString];
            NSData *data =[NSData dataWithContentsOfURL:[NSURL URLWithString:filePath]];
            NSString *storageFilePath = [self randomFilePathInTime:[filePath componentsSeparatedByString:@"."].lastObject];
              BOOL isStorageState = [data writeToFile:storageFilePath atomically:YES];
              if (isStorageState) {
                  success(storageFilePath,[filePath componentsSeparatedByString:@"/"].lastObject,data);
              }
}