The method of lumped mass was adopted to found dynamic model of multi-DOF system for planetary mechanism in the hybrid electric vehicle.The dynamic equation of vibration system with multi-DOF was transferred to its state equation and transfer function equation with the excitation of time-varying mesh stiffness and gear composite errors.The numerical simulations of the vibration system in time domain and frequency domain were carried out by the method of the 4-steps Runge-Kutta and the mechanical impedance analysis method with MATLAB simulation toolbox.As a result

the dynamic response and behaviors of dynamic load between teeth of the system were acquired.The numerical results indicate that the floatation of sun gear can improve the behaviors of dynamic load between teeth.And the greater the amplitude of stiffness varies

the more heavily the system vibrates.