Abstract: A torsional vibration model formulated by time varying mesh stiffness and composite gear errors was established to predict the dynamic characteristics of the encased differential gear train.The mesh phases of the planet and star gears,elastic deformation and load inertia of planet carrier were also taken into account.A numerical method was applied to solving the large parametrically excited dynamic system.From the numerical results it was concluded that the gear meshing forces of the planetary gear train mainly fluctuated in meshing frequency of the planetary gear train;but the gear meshing forces of the star gear train fluctuated not only at the meshing frequency of the planetary gear train but also at that of the star gear train.A maximal dynamic load occurred at the high frequency resonance.The dynamic load factor of the planetary gear train was larger than that of the star gear train in the work speed area.