For gear system of on-orbit spacecraft, the backlash of gear will be varied when entering and leaving the earth shadow, which would result in the loss of transmission accuracy or even break the system. A nonlinear dynamic model of spur meshing gear pair was established by further considering the alternating temperature effect of space environment. The model also includes the effect of time-varying meshing stiffness, the transmission error and backlash of gear system. Combing the phase diagram, Poincare map and bifurcation diagram, the dynamic behaviors of gear system have been solved and analyzed for both considering and not considering the temperature effect by using 4-5 order variable step size Rung-Kutta method. The effect of changing temperature amplitude and frequency also have been studied for dynamics characteristics of the gear system quantitatively. The results show that the alternating temperature affect the gear dynamics greatly, which lead the system trend towards chaos for different condition. And some disciplines could be found about the structure of solutions between the amplitude and frequency of changing temperature. Some results of this work provides a theoretical guidance for the designing and matching of gear transmission system for special space environment.