We precisely expose a problem occurred during re quantization process in MPEG 2 transcoding. Firstly, we expatiate the origin of the problem;then, we systematically propose a scheme which uses MSE rule to reduce the re quantization errors. In the end of this paper, we formulize the differences between traditionally used method and our proposed method. Theories and simulation results show that our proposed scheme efficiently reduces the re quantization errors while results in higher picture quality.