Abstract: Adding auxiliary frequency control module to the wind turbine generator (WTG) is a solution to the power system frequency security problem caused by the replacement of synchronous generator (SG) by the new energy. The frequency modulation mode using rotor kinetic energy can make the WTG operate at the maximum power point and obtain better economic gain than the power reserve mode. However, the flexibility of WTG output control are not fully utilized. The kinetic energy limit of the rotor and the secondary drop of system frequency are not considered. This paper broke through the traditional framework of virtual inertia and droop control. The optimal strategy of the WTG auxiliary frequency control based on rotor kinetic energy was proposed. Firstly, the WTG output power curve was taken as the decision variable, and the optimal power curve was obtained through optimization. Then, the corresponding auxiliary frequency control strategy was designed to achieve the optimal output power curve. The simulations verify the effectiveness of the proposed strategy, and show that the auxiliary frequency control of the WTG should have better strategies instead of being limited to the simulation of the SG.