原理及步骤：

1、新建一个视频的框架

2、往框架里面添加一条音频轨道和视频轨道

3、把需要拼接的视频片段循环读取出视频和音频部分，并依次插入对应轨道

4、导出视频

代码：

- (void)finalMergeAndExportVideos:(NSArray*)videosPathArray withOutPath:(NSString*)outpath success:(void (^)(void))successBlock{

if (videosPathArray.count == 0) {

return;

}

AVMutableComposition *mixComposition = [[AVMutableComposition alloc] init];

AVMutableCompositionTrack *audioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio

preferredTrackID:kCMPersistentTrackID_Invalid];

AVMutableCompositionTrack *videoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo

preferredTrackID:kCMPersistentTrackID_Invalid];

CMTime totalDuration = kCMTimeZero;

//用来计算视频大小（不需要可以删掉）
CGFloat length = 0;

for (int i = 0; i < videosPathArray.count; i++) {

AVURLAsset *asset = [AVURLAsset assetWithURL:videosPathArray[i]];

NSError *erroraudio = nil;

AVAssetTrack *assetAudioTrack = [[asset tracksWithMediaType:AVMediaTypeAudio] firstObject];

[audioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, asset.duration)

ofTrack:assetAudioTrack

atTime:totalDuration

error:&erroraudio];

NSError *errorVideo = nil;

AVAssetTrack *assetVideoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo]firstObject];

[videoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, asset.duration)

ofTrack:assetVideoTrack

atTime:totalDuration

error:&errorVideo];

//合成的视频方向设置成第一个视频的方向 （不需要可以直接删掉）

if (i==0 ) {

[videoTrack setPreferredTransform:[MediaManager ssfixedCompositionWithAsset:asset]];

}

totalDuration = CMTimeAdd(totalDuration, asset.duration);

length +=  [NSData dataWithContentsOfFile:self.marr_movies[i]].length/1000000.0;

NSLog(@"lenght =============  %f", length);

}

NSString * name = AVAssetExportPresetPassthrough;

//视频如果大于5M就压缩（不需要可以删掉）
if (length > 5) {
name = AVAssetExportPresetMediumQuality;
}

NSURL *mergeFileURL = [NSURL fileURLWithPath:outpath];

AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition  presetName:name];

exporter.outputURL = mergeFileURL;

exporter.outputFileType = AVFileTypeMPEG4;

exporter.shouldOptimizeForNetworkUse = YES;

[exporter exportAsynchronouslyWithCompletionHandler:^{

if( exporter.status == AVAssetExportSessionStatusCompleted ){

//        UISaveVideoAtPathToSavedPhotosAlbum(outpath, nil, nil, nil);
successBlock();

}else if( exporter.status == AVAssetExportSessionStatusFailed )
{
}
}];
}

PS：
1、videosPathArray为本地视频片段url的数组:[NSURL fileURLWithPath:path]，注意别写成了URLWithString
2、里面有视频方向设置，以后有时间再单独写出来 (方法如下)

// 获取优化后的视频转向信息
+ (CGAffineTransform)ssfixedCompositionWithAsset:(AVAsset *)videoAsset {
    // 视频转向
    int degrees = [self degressFromVideoFileWithAsset:videoAsset];
    
    CGAffineTransform translateToCenter;
    CGAffineTransform mixedTransform;

    NSArray *tracks = [videoAsset tracksWithMediaType:AVMediaTypeVideo];
    AVAssetTrack *videoTrack = [tracks objectAtIndex:0];
    
    if (degrees == 90) {
        // 顺时针旋转90°
        translateToCenter = CGAffineTransformMakeTranslation(videoTrack.naturalSize.height, 0.0);
        mixedTransform = CGAffineTransformRotate(translateToCenter,M_PI_2);
        
    } else if(degrees == 180){
        // 顺时针旋转180°
        translateToCenter = CGAffineTransformMakeTranslation(videoTrack.naturalSize.width, videoTrack.naturalSize.height);
        mixedTransform = CGAffineTransformRotate(translateToCenter,M_PI);
        
    } else  if(degrees == 270){
        // 顺时针旋转270°
        translateToCenter = CGAffineTransformMakeTranslation(0.0, videoTrack.naturalSize.width);
        mixedTransform = CGAffineTransformRotate(translateToCenter,M_PI_2*3.0);
        
    }else{
        translateToCenter = CGAffineTransformMakeTranslation(videoTrack.naturalSize.height, 0.0);
        mixedTransform = CGAffineTransformRotate(translateToCenter,0);
    }
    
    return mixedTransform;
}

// 获取视频角度
+ (int)degressFromVideoFileWithAsset:(AVAsset *)asset {
    int degress = 0;
    NSArray *tracks = [asset tracksWithMediaType:AVMediaTypeVideo];
    if([tracks count] > 0) {
        AVAssetTrack *videoTrack = [tracks objectAtIndex:0];
        CGAffineTransform t = videoTrack.preferredTransform;
        if(t.a == 0 && t.b == 1.0 && t.c == -1.0 && t.d == 0){
            // Portrait
            degress = 90;
        } else if(t.a == 0 && t.b == -1.0 && t.c == 1.0 && t.d == 0){
            // PortraitUpsideDown
            degress = 270;
        } else if(t.a == 1.0 && t.b == 0 && t.c == 0 && t.d == 1.0){
            // LandscapeRight
            degress = 0;
        } else if(t.a == -1.0 && t.b == 0 && t.c == 0 && t.d == -1.0){
            // LandscapeLeft
            degress = 180;
        }
    }
    return degress;
}