Grid voltage sag causes typical transients in the generator electromagnetic torque.Thus will result in significant stress on the wind turbine drive-train and reduce the service life of the drive system，especially the gearbox.A combination of different simulation packages，namely FAST（fatigue，aerodynamics，structures and turbulence）and MATLAB/Simulink，are used to model the mechanical and electrical aspects of a5MW doubly-fed induction generator（DFIG）based wind turbine respectively.To reduce the mechanical loads induced by symmetrical grid faults，a linear-quadratic regulator（LQR）based generator torque controller is presented.Simulation results show that the proposed controller can effectively suppress the vibration of drive-train after fault ride through（FRT）.