Abstract: When electromagnetic transient simulation of an actual power system is carried out by PSCAD /EMTDC, the actual power system has to be divided into three parts, namely the internal system, the external system and the boundary system, and the external system network is generally equivalently simplified. In order to overcome the deficiency of the traditional methods effectively, the power flow equivalent method is put forward for such systems whose adjacent nodes need to be equivalent simplified at the same time and that there exist coupling relation. By combining with the network structure features of Chongqing power grid, the equivalent simplification scheme for the generator units, loads, AC channels and transmission lines is established. In comparison with power flow distributions, short circuit currents of key buses and transient behavior variations of power grid before and after the equivalence, it is proved that the proposed equivalence scheme is reasonable and the simplified system is practicable.