It is an inevitable problem that low frequency oscillation exists in power system, which weakens the transmission line transfer capability and threatens power system stability. Unified Power Flow Controller based on Modular Multi-Level Converter is an effective solution to power system oscillation. The main method at present is highly dependent on accurate math model, whose design process is complex and against engineering practice as well. This paper proposes a novel damping strategy, which uses reactive current variation to increase system damping ratio in seek of suppression on system oscillation. This method has advantages of good damping effect, simplified design process, and easy engineering applicability. At last, the simulation analysis based on PSCAD software verifies the effectiveness of the design.