Based on the analysis of both the operating characteristics of doubly-fed induction generator（DFIG）and conventional control strategy of maximum power point tracking（MPPT）, a control strategy based on the constraint factor of amplitude-limit to smooth the output of active power was proposed. The expression and calculation method of constraint factor α was provided.Based on matlab/simulink simulation, a power system with one 9 MW wind farm and two synchronous machines was built. Experiment results have shown that, compared with the conventional MPPT control method, the proposed control strategy can smooth the active power of DFIG and weaken the rotor speed fluctuation of synchronous machines, so thus the grid frequency can be improved.