Accurate segmentation of pathological cell nucleus is the basis of pathological diagnosis,however,the current algorithm for automatic segmentation of mitotic breast cancer cell nucleus is poor.This paper analyze and study the current cell nucleus segmentation algorithm.And a U-shaped network (U_net) based on a combination of attention mechanism and residual structure is proposed to solve the problem of insufficient accuracy of cell nucleus segmentation due to the close morphological characteristics of mitotic and non-mitotic cells.The experiments show that the average pixel accuracy (MPA) and Mean_dice index coefficient of the proposed algorithm are 0.74 and 0.82. Compared with the original algorithm, the training indexes are improved by 11% and 9%,which proves the feasibility of the algorithm in this article.