The properties of magnetorheological fluid and the theory of magnetorheological fluid shock absorber were discussed based on quarter vehicle dynamics model of semiactive suspension. We obtained the correlation between the adjusted damp and the state space variable with the theory of stochastic optimal control. The system performance simulation program for 3 DOF's vechile model was developed by using matlab language.As an illustrative,the road induced random response of a mancarroad model with magnetorheological fluid shock absorber was explored.The results show that the vehicle ride comfort can be improved by the control strategy, and it also has guiding significance for improving the comfort of human body under vibrating vehicle and for improving the vibration behavior of road.