For the automobile engine timing belt system,by using the AVL-Excite Timing Drive to carry out a periodic dynamic simulation analysis. The variation law of the load when the loose and tight of belt contact with pulley is studied. The results show that dynamic tension and its fluctuation of the belt between the idler and the exhaust camshaft pulley are the largest. And the peak mechanism of the belt toot load is studied. Based on the analysis of the vibration performance of the main vibration source,the system parameters are optimized. The results show that the vibration of the main vibration source is reduced and the transmission error of is reduced.