Abstract: Constructing the equivalent inertia model of wind turbines is a key basis for quantitative analysis and optimal control of wind farm transient support, especially in the scenario of high-proportion renewable energy power grid with reduced of inertia and degraded frequency characteristics. However, the existing model research often focuses on several links in combination with the analysis goal, but does not establish a complete model of the whole links, and does not reveal the relationship between the reducedorder model and the calculation accuracy at different time scales. Firstly, taking direct-drive wind turbine as the object, based on the comparison of the equivalent inertia of wind power and synchronous machine in mechanism and implementation mode, the dynamic influence of mechanical, control and electrical links on the equivalent inertia of wind power is analyzed, and a full-order frequency response model of the wind turbine is established. Then, based on the singular perturbation theory, the equivalent inertia reduction model considering the dynamics of different time scales is derived, and the relationship between the error and the time scale of the model is derived by applying the Vasilyeva theory. Finally, an electromagnetic transient simulation case is used to verify the effectiveness of the full-order mechanism model and the accuracy of different reduced-order models.