In this paper, new sprocket tooth profile is developed to reduce polygonal action and meshing impact under high speed. The new conjugated profile is derived by modifying involute profile to guarantee that the moving distance of chain is equal to the arc length of pitch circle that sprocket rotates at the same time and the centre line of chain at tight side is tangent to the pitch circle always. An asymmetrical modification method for the sprocket tooth profile is also proposed. The multi-body dynamic model of timing mechanism in engine with the intake and exhaust sprockets is developed. The fluctuation and meshing impact of chain are analyzed under different rotating speeds. The results show that new developed sprocket profile can efficiently reduce meshing impact and friction of chain. The stability of chain transmission under high speed can be improved.