Abstract: The equivalent model is one of the most important mean to analyze the operation of power grid with wind farms. In order to reduce the difficulty of the equivalence of wind farm and to improve the efficiency of the grouping of wind farm, fuzzy C-means (FCM) algorithm is applied to equivalence of wind turbines in the wind farm based on the monitored operational states of wind turbine generator system (WTGS). Firstly, the output active and reactive power, voltage and current of WTGS are adopted as grouping index, and the wind farm grouping problem is transformed into a clustering problem according to the given number of equivalent WTGS. Secondly, the fuzzy membership function indicating WTGSs category and the objective function of FCM clustering algorithm are given. The clustering result is obtained based on the optimal cluster center and fuzzy membership matrix solved by iterative solution, and FCM is a simple algorithm with good convergence. Thirdly, according to the grouping result, the multi-machine equivalent model of the wind farm is achieved through the equivalence of different groups of WTGS. In the end, the validity of the approach is verified through simulation. The clustering index in FCM is more practical than other index. For given number of equivalent WTGS, the calculation of FCM is simpler and the accuracy is higher too. This method is very suitable for the equivalent model of wind farm in practice.