For a large-scale wind farm with wind turbine driven doubly fed induction generator（DFIG）， a new equivalent modeling method is proposed. This method uses the DFIG pitch control action as the grouping rule of wind turbines. By extracting eigenvectors that reflect pitch control action， it is used as input of support vector machine（SVM）. The wind turbines are dynamically grouped， and the same group wind turbines are aggregated. The calculating methods of equivalent model parameters are provided. Thus a wind farm model represented by three equivalent wind turbines is obtained. Compared with the traditional equivalent modeling methods， the results show that the proposed three-machine equivalent model more accurately describes the dynamic characteristics of a wind farm at the grid connection point.