Abstract: In order to enhance the air compensating effect of the secondary air injection system, a model for a motorcycle engine is set up and the performances of the model JS125-12A are calculated and analyzed by using the engine performance analyzing software package of AVL BOOST. On the basis, the model of the secondary air injection system is set up, and the main design factors of the secondary air injection system including the intake position, the diameter of the intake pipe,the intake angle and the valve spring rate, are forecasted and optimized. The results reveal that the secondary air injection system matches engine well when the distance between the secondary air injection point and the exhaust port is (30 mm,) the diameter of the intake pipe is (8 mm,) the intake angle ranges from 30°～60° and the valve spring rate ranges from 100～500 N/m.