An output feedback variable structure controller was designed for automotive semi-active suspension with electro-rheological (ER) damper. Sliding surface matrix has been selected in accordance with Hurwitz stable judgement of sliding mode motion equation, the control law was completely decoupled from external excitations. The isolated mass response of many kinds of excitations and the robustness of semi-active suspension system with respect to parameter variations were simulated. The simulation results indicated that the performance of semi-active suspension system designed with varying structure control methods are better than those of optimally passive damping, semi-active suspension system has self-adaptability with respect to external disturbances and good robustness with respect to system parameter variations.