Taking 2K-H differential gear train as study object, dynamic equation of torsion vibration was established with influence of time-varying meshing stiffness. The virtual-prototype with nonlinear meshing force was modeled using Virtual.Lab Motion software. Gear contact force was simulated, and it was verified by theoretical data. The reason of meshing vibration is analyzed. The results show that time-varying meshing stiffness is the main excitation of gear system, and gear system is vibratory although the input and output are stable, and the basic frequency is meshing frequency. This research lays foundation for strength checking, optimum design and fatigue life prediction.