As the drive train torsional vibration may be stimulated by the huge change of electromagnetic torque caused by grid fault, the flexible drive train is modeled using spring-damping-mass modeling method firstly, the analytical expression of the natural oscillation frequency and damping coefficient are obtained, and the flexible drive train is typical under damped system is revealed. Then the ineffectiveness of the change of generator torque and high frequency torque pulse control is illuminated by analyzing characteristics of the generator electromagnetic torque during grid fault. On this basis, the torsional vibration suppression strategy of virtual variable damping is proposed, which is worked by implementing additional electromagnetic torque to increase in the drive train damping equivalently during the grid fault. The Bladed and Matlab co-simulation results strongly support the theoretical analysis and verify the proposed control strategy.