Abstract: With the improvement of power electronics of power system year by year, more and more new energy sources (such as wind power, photovoltaic, etc.) have been connected to AC power grid through VSC. The dynamic interaction between new energy stations and power grid has triggered many new Sub-synchronous Oscillation events, which poses a serious threat to the safe operation of power system. When studying the interaction between new energy stations and power grid, the "bottom-up" method used for modelling each VSC in the station may cause the problem of "dimension disaster", and the analysis results are difficult to recognize. Therefore, it is necessary to study the dynamic equivalent modeling theory of wind farm. This paper proposed an aggregation modeling method of grid connected VSC based on structure preservation. The sub-synchronous oscillation caused by grid connected VSC was analyzed, which is mainly related to the AC current control scale. The aggregation model of grid connected VSC under the AC current control time scale was established, which is suitable for the small signal stability analysis of grid connected VSC under the AC current control time scale, reflecting the relationship between the overall characteristics of the wind farm and the characteristics of a single internal fan. The simulation verifies the effectiveness of the aggregation model.