Based on the dynamics of the gear system and Lagrange equation, the coupled dynamic model of the high speed planetary gear transmission system is established by using the concentrated parameter method. The model takes into account the floating amount of components, the varying stiffness of the sliding bearing over time, and the overall meshing error. The variation of load sharing coefficient IS obtained by numerical analysis method, and the influence of different working conditions and sum error on load sharing characteristics is studied, which provides guidance for the analysis of load sharing characteristics of high speed planetary gear transmission system.