A novel UPFC control method based on current-injection method is proposed. In the series side the active and reactive power control of the transmission line is realized based on current prediction, and the reactive injected current of the shunt side is used to control the bus voltage. Based on the user program interface (UPI) function of power system analysis software package (PSASP), the power flow control model of UPFC is established by user program, written in C++ language, which runs alternatively with load flow (LF) program in order to activate the control of UPFC. The accuracy and validity of the proposed model are verified through case-study. No change of the bus type is needed in the proposed model, which makes the control flexible. Furthermore, the concept of current injection is instructive for the access of flexible AC transmission system (FACTS) controllers to other business software.