In order to perform electromagnetic transient analysis on a part of a large-scale power system by PSCAD, the whole system has to be divided into two parts, namely the internal system and the external system, and then the equivalent simplification of the external system is carried out. Because the equivalent parameters of the external system obtained by Ward equivalent method contain the ground admittance parameters of the transmission line simulated by π equivalent circuit of the internal system, the original system power flow cannot be reproduced after the external equivalent system is spliced with the internal system. For this reason an equivalent method for regional power network based on boundary branch is proposed, by means of the boundary branch with minimum impedance between internal system and external system, the equivalent parameters of the external system obtained by Ward equivalent method will not contain any parameters of the internal system.The proposed method also overcomes the limitations of PSD-SCCP which can only calculate the equivalence of the whole system. And then the specific steps of realizing the equivalence method of external network based on boundary branch by PSD-SCCP are given. Theoretical analysis and results of an actual system simulation show that this method has a high veracity and practicability.