In order to analyze the influence of external and internal excitation on the dynamics of the timing gear transmission system, the dynamic model of the timing gear transmission system was established by using 〖BF〗AVL_EXCITE〖BFQ〗 Timing Drive software.The effects of the external and internal excitation on the dynamics,including internal excitation of load torque,crankshaft speed fluctuation and the external excitation of tooth clearance were analyzed.The angular velocity and meshing force curves of gears were compared and studied under different loads,when considering or not considering the crankshaft speed fluctuation and under different backlashes. The results show that the peak position of angular velocity and meshing force curve is close to the peak position of load, and the peak value is greatly influenced by the peak value of load.When the camshaft gear needs torque output, the meshing force produced by the driving side of the gear is significantly higher than that produced by the backlash side. The existence of crankshaft speed fluctuation makes the dynamic results change greatly,so the crankshaft speed fluctuation is a factor that must be considered in order to carry out the dynamic analysis more accurately.With the increasing of gear clearance,the peak value of angular velocity and meshing force are increased correspondingly,the fluctuation of angular velocity is increased,and the vibration and impact in the meshing process becomes more obvious.However,too small side clearance design will increase the processing difficulty and cost,so it is necessary to select the side clearance reasonably.