Abstract: Analyzes the meshing stiffness using finite element method, and proves that there are several indispensable harmonic having great effects to the system. Then considering the time varing meshing stiffness and backlash gives the dynamics differential equation. Based on this equation and according to numerical calculation it displays the phase diagram and Poincare mapping diagram at different driving frequency and damping ratio. The results show that the systems cycles and collision vibration characteristics will be of complex changes with the excitation frequency and damping ratio.