With the continuous development of new power systems, the penetration rate of wind power generation in various regional power grids has increased to a high level. However, large-scale wind farms generally cannot provide inertia support. Even after the introduction of virtual inertia control for wind turbines, there is still a lack of accurate estimation and analysis of virtual inertia and its changing dynamics in wind farms. Therefore, an effective inertia estimation method for a wind farm is proposed from the virtual inertia control mechanism of the fan and the influence of external wind speed. In this method, the mechanical-energy loss caused by the offset of wind turbine working point in the virtual inertia response phase is first considered. On this basis, the concept of effective inertia is proposed and the relationship between effective inertia and wind speed is studied. Second, the randomness and correlation of wind speed in a wind farm is considered. A spatial and temporal distribution model of wind speed is proposed based on the Copula function, wake effect and propagation delay. Finally, the estimation of the spatial and temporal distribution of the effective inertia of the wind farm is obtained. A simulation example is constructed based on the actual wind speed and output data of a wind farm in Gansu Province. The simulation results reveal that the proposed algorithm can effectively estimate the effective inertia of each wind turbine in a wind farm and the time-varying characteristics with varying wind speed. It can reflect the virtual inertia support capacity capability of each wind turbine in real time and provide a data basis for mastering the frequency immunity of the system.