Abstract: Aiming for reducing the in-plane peak displacement responses of wind turbine blades subjected to ultra-strong wind load,in this paper,a new electromagnetic inertial mass damper（EIMD） is applied to vibration control of wind turbine blades and the corresponding semi-active control strategy is proposed by using a new quasi-sliding mode active control algorithm.In order to avoid excessive chattering effect of the sliding mode controller,the quasi-sliding mode control law is deduced on the basis of the saturation function.The effectiveness of new quasi-sliding mode control algorithm is demonstrated by application to a real-life NREL-5 MW wind turbine.The simulation results show that the peak of the tip displacement of the wind turbine blades using new quasi-sliding mode control is significantly lower than that under uncontrolled conditions,and the tip displacement responses using semi-active control are virtually the same as those using new quasi-sliding mode control.Moreover,the robustness of the proposed control method is verified by the 5%,10% and 20% reduction of the original stiffness of the wind turbine blades,respectively.