Dynamic simulation of gear transmission system is carried out. A bearing-rotate coupled dynamic model, which considers the influence of friction force of gears-meshing on the system, is established. According to the torsional mesh stiffness of different contact position, the time-varying nonlinear differential equation of the system is solved by using Matlab software. The dynamic response of spalling system is simulated by torsional power. The differentia is obtained by comparing dynamic response of that with and without spalling systems, which can make a prediction for the dynamic performance of gear transmission system and supply theoretic elements for fault diagnose.