Ultra-precision active vibration isolation system protects ultra-precision equipment from the transmission of external vibration, and suppresses vibration incurred by internal structures. In order to obtain the ideal skyhook effect, the sliding mode controller with adaptive feedforward compensator is proposed in this paper. With the controller, the vibration system can obtain the desire performance even when the system is under model uncertainties and significant direct disturbance. A series of simulations are carried out to verify the effectiveness of the controller.