Abstract: The time consuming of short circuit current calculation increases with the development of the power grid scale, and the requirement for computation software is enhanced. An external equivalent method is introduced for calculation of the short circuit current in order to improve the calculation efficiency. Firstly, the node admittance matrix of the whole power network is equivalently simplified to form the additional elements of the boundary region. And then a simplified power network composed of the study region and boundary region is constructed to calculate the short circuit current in the study region. The validity and efficiency of this proposed algorithm is theoretically analyzed and verified on the IEEE 39-node power system. Finally, the North China power grid is used as an example for external equivalent simplification to form the short circuit calculation data of a certain area, which greatly reduces the scale of the power system and improves the calculation efficiency of short circuit current.