With the development of wind energy, it is necessary to develop equivalent models to represent dynamic behaviors of wind farms in power systems. The equivalent wind method is investigated for the aggregation of doubly-fed induction generator wind turbines. The detailed procedures for the calculation of equivalent wind are analyzed. The necessity of classifying incoming winds is shown. To improve the performances of the method,incoming winds are classified according to mean wind speeds and positive/negative semi-variances of wind speeds,and a group of turbines with similar incoming winds are aggregated together. The effectiveness of the method is verified through simulations in MATLAB/Simulink.