By using lumped-mass method, the dynamic model of planetary gear transmission with floating sun-gear was established; the time-variant stiffness and meshing error were considered in the model. The motion differential equation of the model was presented and solved by Runge-Kutta numerical integration method with variable steps. Dynamic time-domain responses, dynamic loads and load sharing characteristics of the high contact ratio planetary gear transmission and general planetary gear transmission were both investigated for comparison. The results indica-ted that the high contact ratio planetary gear transmission had lower vibration amplitude, smaller dynamic load and dynamic load-sharing coefficient. So the high contact ratio planetary gear drive had better performance with smoot-her transmission, lower noise, and its load distribution is more uniform.