Unified Power Flow Controller (UPFC) contains multiple SISO controllers. These controllers run separately to good effect. When they jointly run, there are negative interactions between each controller. This paper analyzes working relations between each controller of UPFC. The results show that negative interactions, caused by reactive power flow control, relative to the degree of coupling between active and reactive power controllers. Then, depending on the cause of negative interaction, we design current decoupling controller using hierarchical structure. The outer controller provides reference signal for inner controller. The inner controller applies decoupling control strategy to realize the decoupling function. Finally, this paper establishes dynamic model of UPFC in PSCAD, and compares the simulation results of conventional power flow controller and trend decoupling controller. The results verify the superiority of trend decoupling controller, which can effectively solve negative interaction problems caused by reactive power regulation.