Aiming at the complicated internal structure of abdomen and the mutual infiltration between organizations, an improvement of level set method is used to extract the 3D abdomen magnetic resonance image segmentation of liver area. Level set method is easy to leak in weak edges. Therefore, this problem is solved by preprocessing threshold segmentation. Certain interference in further processing is reduced and the edges is more clear through remapping the intensity of pre segmentation results, and then the improved level set method is used for further segmentation. Experiments proves that the method is superior to traditional level set methods. The leaking problem of weak edges is reduced, and the desired segmentation result is obtained. The applicability of level set is expended.