In particular practical engineering application, the parallel side transformers of UPFC are integrated to the low voltage buses. Due to the special structure of UPFC, it is difficult to analyze the impact of UPFC on low voltage buses, neighboring substations and transmission lines with existing model. A three-bus power injection model of UPFC is proposed in this paper in order to solve this problem. Based on power injection method, the mathematical expression of a three-bus power injection model of UPFC is derived. A three-bus power injection simulation model is established by using the User Defined (UD) function of Power System Analysis Software Package (PSASP). In this model, the UPFC parallel power injects into the low voltage buses, which overcomes the limitation of existing model since they cannot reflect the impacts of UPFC on low voltage buses, connected substations and transmission lines. This three-bus power injection model has been put into practice successfully in the simulation of the Nanjing UPFC project. Simulation results reveal the accuracy and reliability of this UPFC model. This work is supported by National Natural Science Foundation of China (No. 51407028) and Natural Science Foundation of Jiangsu Province (No. BK20140633).