Abstract: Methods for modifying watershed algorithm are proposed to resolve over-segmentation problem of watershed algorithm in region-based segmentation process. It adds preprocess, adopts mended process watershed algorithm. Since the whole process is based on local watershed algorithm, it reduce the storage space. The experimental results showed that the improvements can segment the blood cells in medical images effectively. It is successful to solve the problem of overlapped objects with the high speed, the appropriate result and the automatic mode.