To research the mechanical and influence factors of noise of valve train of engine, noise-distinguishing test is taken with one certain 125 mL engine valve train. The result reveals that the valve train noise mainly results from impacting between rocker and valve stick, impacting of valve seating. With the way of model set up and simulating calculation of kinematics and dynamic of valve train, the influence factor of rocker impact force and valve seating acceleration of valve train is studied, which reveals that the acceleration of valve and impacting force between rocker and valve stick are mainly relied on cam buffer segment lift, cam buffer segment wrap angle and coefficient of accelerating segment of cam buffer segment. Choosing these parameters reasonably is helpful to decrease the acceleration of valve and impacting force between rocker and valve stick, and then reduce the noise of valve train.