The improved "SEN Transfomer" (IST) is an electromagnetic unified power flow controller composed of three-phase double-winding transformer and on-load voltage regulator switch, which has more flexible power flow control ability and larger power flow control range than ST. Firstly, the topological structure and basic working principle of IST are introduced in detail, and the power injection model of IST is deduced. According to the basic siting principle of IST and the calculated sensitivity coefficient of power flow, the installation position of IST is selected to improve the regulation ability of active power flow. Finally, the proposed method is validated in the IEEE 14-bus system. The results show that the distribution of active power flow of the system is obviously improved after IST is installed on the branch road calculated by this method.