A semi-active static output feedback variable structure control (VSC) strategy is presented to control a two-stage vibration isolation system in this paper. A continuous output feedback VSC controller which utilizes the measurements from a limited number of sensors installed at strategic locations is designed and a bypass electrorheological (ER) damper is applied to achieve the best control effect more rapidly and accurately. The determination of sliding surface in terms of the Routh-Hurwitz stability criterion is combined with continuous controller design such that the control law is completely decoupled from external excitations, which are hard to measure or estimate. The self-adaptability of the vibration isolation system with respect to external disturbances, the robustness of the control method with respect to parameter variations and the effectiveness of vibration isolation are demonstrated by numerical simulation results. It showed that the designed semi-active static output feedback VSC strategy realized by the ER damper can achieve better performance than those of optimally passive damping and maximum damping variety even if system parameter uncertainties exist.