Abstract: With the gradual decline in the frequency regulation capability of the power system with a high proportion of renewable energy and a high proportion of power electronic equipment, frequency security issues have gradually become prominent. Power generation equipment with renewable energy is connected to the grid using power electronic interfaces, and is generally controlled by maximum power point tracking, so it is considered not responding to system frequency changes. However, the doubly-fed wind turbine is directly coupled to the grid through the stator electrical circuit, which will produce a complex response to frequency disturbances under the joint action of electromechanical coupling and rotor circuit decoupling control. Existing research has noticed this characteristic, but there is still a lack of a model that accurately describes the frequency response process of doubly-fed wind turbines and an in-depth analysis of the frequency response characteristics and mechanisms. Therefore, this paper proposes a reduced-order frequency response model of doubly-fed wind turbine that does not depend on the grid-connected point voltage information and can be directly integrated into the power system frequency response model. Using this model, it is revealed that the frequency response mechanism of doubly-fed wind turbines is the cascade coupling effect of the phase-locked loop and the outer loop, and it is proven that its frequency response does not belong to the category of inertia response. Finally, the influence of doubly-fed wind turbines on frequency dynamics of the power system is analyzed.