To simulate the three sequence impedance accurately, a multi-port external system equivalent circuit model is proposed as well as the realization method which is proper for electrometrical and electromagnetic hybrid simulation. The method is based on the current compensation principle. Through calculating the current injected by the external system, the method settles the problem of solving the nonsymmetrical admittance matrix. The simulation results show that the method can reflect the three-sequence equivalent impedance and could be easily expended to multi-port. The method is proper for the simulation analysis after an unbalanced fault occurs in electromagnetic side. This work is supported by Fundamental Research Funds for the Central Universities (No. 2015XS22).