A six degrees non-linear dynamics model of a gear system with meshing beyond pitch point is established considering the time-varying meshing stiffness, time-varying friction, load distribution among teeth and backlash. Time-varying meshing stiffness is calculated by material mechanics method, and time-varying friction coefficient is calculated by elasto-hydrodynamic lubrication (EHL) theory and compared with that of the Coulomb friction model. Moreover, the system differential equation is obtained and solved by Runge-Kutta numerical integration method. At last, dynamic response and phase diagram are obtained, and dynamics characteristics of the system are analyzed.