According to mathematical morphology, a new image segmentation method based on regionally minimal cost watershed transform is introduced. Firstly, local region needing watershed was segmented after erosion、dilation and edge detection by canny. Secondly, the local region of the picture is processed applying the methods in this paper. And in the last of this paper many rules solving over-segmentation are introduced. Experiments show that the new algorithm′s time is low and the method in this paper has the advantage of giving accurate contour of object.