The dynamic model of the spur planetary gear was established. The mesh nonlinear between gear teeth was described by the spring?damper?clearance?mesh error. The expressions of the time?varying meshing stiffness and the time?varying meshing error with the rotation angle of the planetary gear as a variable were proposed which solved the problems of describing and resolving the dynamic model of the planetary gear under the variable speed. Then, by solving the dynamic models, the effects of the important parameters such as rotational speed, tooth side clearance, meshing error and the load on the mechanical property of planetary gear were analyzed.