In order to meet the needs of project planning and design, the equivalent circuit of the Unified Power Flow Controller (UPFC) based on the Modular Multilevel Converter (MMC) is analyzed. Then, an electromechanical transient simulation method based on PSS/E is proposed. The power injection model of the UPFC and the alternating iteration method are used in the load flow calculation. To realize transient simulations, the UPFC is represented by current sources. The direct current system, controllers and the modulation of the UPFC are taken into consideration. By integrating this method in PSS/E, both a small system and a real system are tested. The results show that the proposed method, which is suitable for real power systems. This work is supported by Science and Technology Project of State Grid Jiangsu Electric Power Co., Ltd. “Research on Long-term Dynamic Stability Characteristics and Simulation Technology of Flexible Transmission Equipment including UPFC”.