实例：

OpenCvSharp.Size dsize = new OpenCvSharp.Size(1920, 1080);
FourCC codec = FourCC.XVID;
VideoWriter write = new VideoWriter("videoPath/videoName.avi", codec, 20.0, dsize, true);
DirectoryInfo directoryInfo = new DirectoryInfo("imagesPath");
FileInfo[] fileInfoList = directoryInfo.GetFiles();
foreach (FileInfo di in fileInfoList)
{
   OpenCvSharp.Mat mat = Cv2.ImRead(di.FullName);
    write.Write(mat);
}
    write.Release();

写出后发现文件体积很小，基本不到1Kb，解决方法：

The MatType is CV_8UC4.
I convert it into CV_8UC3.
And then this problem has been solved.
Now the video can be played.
Example code:
Cv2.CvtColor(mat, mat, ColorConversionCodes.BGRA2BGR);