UPFC(unified power flow controller)is the most advanced flexible AC transmission system (FACTS) technology, but the conventional cross decoupling control method in the dq rotating coordinate system still exists coupling between the active control and reactive control loop. The couple characteristics between active power and reactive power in voltage d-q rotating frame are analyzed, and a novel control strategy in polar frame is proposed, which adopts active power-phase, reactive-amplitude decoupling control method of the power. The detail simulation model of HVDC transimission system is built, which includes UPFC. Simulation results comparison shows that the proposed strategy effectively reduces the couple between the active and reactive power control loop, enhances the UPFC system power-flow control performance especially under solely active power or reactive power control scenario.