Abstract: When studying the impact of large-scale wind farm connected to the power system on the power grid,the large-scale wind farm is often aggregated into an equivalent wind turbine to simplify the power system calculation model. But any equivalent method will produce equivalent error. In order to study the single unit aggregation equivalence process of wind farm and the source of error,there are detailed theoretical derivation and analysis based on the mathematical model of wind turbine. Then conclusions are obtained from the mechanism analysis that the rationality of single unit aggregation equivalence and the reasons of error. The correctness of the analysis conclusion is verified by simulations.