In conventional vector quantization (VQ) method, the reachable best PSNR is implemented by full search or its equivalent method when codebook is fixed, To achieve an even higher PSNR by fixed codebook, a post-processing VQ method is proposed in this paper. This method firstly checks how well the VQ matching is for a whole image and locates some bad matching image blocks definitely. Then it partitions a bad matching image block into two partial blocks further by a certain pattern and encodes them once again using the same fixed codebook but two different codes partially. It makes total distortion for current bad matching image block smaller. At the receiving end, current bad matching image block is synthesized by two codes partially according to adopted partitioning pattern so as to realize a better representation. For 10 gray-level standard images of size 512x512 with different detail, 0.3 dB ~ 1.2 dB PSNR improvement can be reached individually by just partitioning 5% worst matching image blocks at the cost of less than 1 bit degradation of equivalent code length. Moreover this improvement is basically for edge portion of an image. Therefore, subject assessment can be made much better at the same time.