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The fast response characteristics of HVDC system can be used efficiently to damp inter-area low frequency oscillation without additional equipments. Phasor measurement unit (PMU) based on Global Positioning System (GPS) is new measurement equipment with high accuracy. It can obtain the phasors of bus voltages, especially the voltage phase angles, so it is possible to utilize PMU to improve control effect. In this paper, a novel HVDC supplementary controller based on PMU is proposed. The input signal of this controller is the voltage phase angle difference between two areas, which can reflect the inter-area low frequency oscillation characteristics effectively. Small signal stability and transient stability of a two-area, four-machine AC/DC test system are studied. The analysis results show that the proposed controller is capable of damping the system's low frequency oscillation and enhancing the AC system's transfer capability over a wide range of operating conditions.