Abstract: A mathematical model of semi-active suspension with electrorheological(ER) damper is built up,and its kinematic and state space equations are established on the basis of a 1/4 dynamic model.The ER damper has been simulated with two extensive on-off control methods.Simulation results demonstrated that the on-off control methods can effectively control the dynamic deflection of the suspension and that of the tire,and as a result diminish the body acceleration to some extent,improving the riding and handling characteristics.The on-off control can only supply non-continuous control signal but not the optimal continuum control signal.As a whole,the on-off control method has an outlook on application owing to its simplicity and effectiveness.