his paper designs the UPFC dynamic model after a briefly review to UPFC operating principles. Firstly, three-phase bridge full-controlled circuit is used to compose the converters, in which IGBT are employed as the switch components. Gate firing signals of IGBT are obtained using SPWM method. Secondly, whole UPFC model, together with a simple PI control rules, are accomplished after analyzing the transmission active and reactive power characteristics. Finally, the capabilities of the UPFC on enhancing transient stability, maintaining bus voltage and damping system oscillation are examed by simulation. Result graphics illustrate the unique capability of the UPFC to control the real and reactive power on a transmission line, while also regulating the local bus voltage simultaneously or independently.