Because the white blood cells, trichomonas and epithelial cells in the samples under the leucorrhea microscopic examination have the characteristics of fluidity, and they are on the two or more focal planes. In order to solve the problem that it is difficult to obtain the focal plane of interest, an automatic focusing definition judgment algorithm based on visual tracking is designed. The algorithm is divided into two focusing processes, coarse granularity and fine granularity. Coarse-grained focusing is mainly used to fix the focal plane near the true focal plane. Fine-grained focusing starts near the end position of the coarse-grained focusing. Fine-grained focusing is realized by moving the camera gradually, extracting the pathological basis object such as white blood cells and candida albicans, performing definition recognition judgment. Finally the clearest focal plane where the objects of the pathological basis are located is obtained. Through 1556 actual tests of the automatic focusing process, the algorithm can effectively improve the success rate of automatic focusing. By achieving automatic focusing, the cost of laboratory manpower is saved.