Adding the auxiliary frequency control function to the wind turbine generator (WTG) is a solution to the frequency security problem of the power system caused by the replacement of the synchronous generator (SG) by the WTG. The auxiliary frequency control using rotor kinetic energy is an economical scheme because the WTG still runs at the maximum power point during normal operation. In this paper, the functional optimization model of the auxiliary frequency control strategy of WTG is established. The optimal auxiliary frequency control strategy is obtained by solving the model numerically. As for the practical realization of the control strategy, the coordination of the auxiliary frequency control with the maximum power point tracking (MPPT) control is studied. The practical auxiliary frequency control strategy is modified to adapt to different power disturbances in the system, and the parameter setting method is also proposed. The sensitivity of system frequency to control parameters is studied. Finally, the simulation results verify the effectiveness and practicability of the proposed control strategy.