The ATP-EMTP is proposed for use as a transient stability simulation tool for large power systems. The purpose is to simulate the action process of power electronic equipment beyond the capability of the electro-mechanical simulation program and the asymmetrical operation of the nonlinear components model and system. An analysis of the model of a different fixed series compensator shows the necessity of using ATP-EMTP in transient stability simulation. Comparisons have been made between the power flow and transient stability results of Subei 220 kV and the above power system obtained from ATP-EMTP and PSASP, respectively. The results show that it is feasible to use ATP-EMTP as an excellent tool for large power system stability simulation.