Doubly-fed induction generator (DFIG) is an asynchronous generator with three-phase excitation winding structure. It can produce voltage in constant frequency when its rotor runs in invariable speed. In this paper, the Park's Transformation is used to infer the model of DFIG in the dq coordinate system, in order to decouple the control of active and reactive power. By using grid voltage directional vector control with feedforward compensation, the strategy realized constant frequency output of DFIG in variable speed and power decoupling control. Simulation results show that the control strategy has a good effect on obeying the power command, and achieves constant-frequency output under varying wind speed.