Abstract: The nonlinear dynamic equations for a two-degree-of-freedom bilinear suspension system of vehicle are established. The amplitude-frequency response function is derived by means of KB averaging method. The results shows plenty of nonlinear characteristics. The effects of nonlinear spring stiffness, coefficient of damping, roughness of ground and spring clearance on vibration curves are further discussed. The amplitude-frequency response curve obtained in theoretical analysis is in fair agreement with the one obtained in numerical analysis. The results may supply theoretical bases for parameter recognition of bilinear vibration, the analysis and optimal design of stabilized region of vehicle suspension systems.