The utility model discloses a differential mechanism installed on a front drive automobile. The differential mechanism comprises a planetary gear and a planetary shaft matching with the planetary gear. According to the differential mechanism, a milled flat position structure is arranged on the planetary shaft along the axial direction of the planetary shaft, and a flat position is arranged on the planetary shaft; during a gear rotation process, on one hand, an oil film can be formed between friction surfaces which can be fully lubricated, and abrasion is relieved significantly; on the other hand, lubricating oil can get in conveniently, friction pairs are well cooled, and oil viscosity decline caused by high temperature arising from friction is avoided. The structural design is based on a planetary shaft (a design of a cylindrical structure) in the prior art, the side of the periphery of the planetary shaft is milled to be a plane structure used for increasing fit tolerance between the planetary shaft and the planetary gear. Therefore, smoothness degree of lubricating oil flowing is increased. Meanwhile, lubricating oil can form an oil film conveniently, cooling effect on a planetary gear train is improved, reliability of a differential mechanism of a front drive automobile is improved, and driving safety is enhanced.