A multibody dynamic model for helical planetary gear train (HPGT) is proposed and the dynamic performance of the HPGT system is analyzed. The free vibration characteristics, steady-state dynamic responses and effects of design parameters on system dynamics are investigated through numerical simulations. The free vibration of the HGPT is classified into 3 categories according to modal properties such as eigenvalue multiplies, modal coordinates of central components and coordinates ratios of planets. The classified vibration modes are demonstrated as axial translational and torsional mode (AT mode), radial translational and rotational mode (RR mode) and planet mode (P mode) followed by the characteristics of each category. The simulation results agree well with those of previous discrete model when neglecting the component flexibilities, which validates the correctness of the present multibody dynamic model. The steady-state dynamic responses indicate that the dynamic meshing forces fluctuate about the average static values and the time-varying meshing stiffness is one of the major excitations of the system. The parametric sensitivity analysis shows that floating sun gear and reducing circumferential error of planets are two effective solutions for vibration control of the HGPT.