The dynamic model of planetary gear system was built up with the consideration of the running characteristics of variable speed and variable load of wind turbine transmission system. The expression of dynamic factor which described by external change load and deformation vibration was studied. Then, the response characteristics and dynamic load of planetary gear system of wind turbine have been studied with the consideration of changing external load and time-varying stiffness, mesh phase and nonlinear stiffness of bearing. The results show that: the frequency response of system reflects the low frequency component of external load. The influence of external incentives on sun gear and planet gear is very significant, and the distinguish between low frequency vibration and high frequency vibration is obvious. The flexibility of system has an obvious impact on dynamic load of bearing. The results lay a foundation for strength design and fatigue analysis of wind turbine gearbox.