From the viewpoint of damping the inter-area low frequency oscillation and decreasing the shaft torsion oscillation of the wind turbine drive train， an ancillary damping control strategy is proposed for wind farms with doubly-fed induction generators（DFIG）. Firstly， the transient state models of DFIG considering the drive train shaft tensional flexibility and control strategies are presented. Secondly， ancillary damping control strategies considering the power signals in transmission lines are developed on the reactive power control loop. Finally， an example of two-area four-machine power system of IEEE integrated with wind farms with DFIG is presented， with three-phase short-circuit fault on the transmission lines and the small disturbance of active power on the synchronous generator， dynamic performances of additional damping control are simulated by using different gains on the active power loop and reactive power loop respectively. The simulation results show that the power oscillation in the transmission lines is reduced by using the ancillary damping control strategy on the active power loop and reactive power loop， and that the shaft torsion torque oscillation of the wind turbine drive train cannot be increased by the reactive power loop with this strategy.