To perform electromagnetic transient analysis on a part of a large–scale regional power network by ATP-EMTP, it is necessary to divide the regional power network into internal system and external system so as to make the external network simple for the restriction of software. To simplify the original system while retain its short circuit current characteristics, a practical equivalence method is proposed in this paper using the short circuit calculation by Power System Analysis Software Package (PSASP). Using the presented method, the equivalence of northwest power network operating at the large mode of the summer 2017 is completed. And the electromagnetic transient model of the equivalence network of Urumqi city is established in ATP-EMTP. By comparing the power flow and the short circuit level in the network before and after the equivalence, the presented method is proved to be correct and feasible.