The control of a semi-active suspension of the full-vehicle model has been investigated. According to the Lagrange equation of the second type, a special Lagrange equation of the second type for the semi-active suspension of the full-vehicle model is obtained. Then the equation of motion and its state space form for the system are established, considering the comprehensive coupling of the vertical motion, the pitch motion and the roll motion of the sprung mass. The semi-active suspension of the full-vehicle model is controlled with some on-off control laws, using four MR dampers as actuators for the system. The simulation results demonstrate that the on-off control is just of little use for the control of the vertical acceleration and the roll angular acceleration of sprung mass, the pitch angular acceleration is even deteriorated. However, it can effectively control the dynamic deflection of the suspension, the dynamic deflection of the tire and the vertical acceleration of unsprung mass. Furthermore, the rear suspension is much better controlled than the front one. Here, the No.4 control law is the best on-off control law and easy to be realized. The results can be the reference of the more advanced control law for the semi-active suspension of the full-vehicle model.