High voltage direct current (HVDC) solves the problems of voltage reducetion, line loss imcrement and reactive power compensation in long-distance transmission, but it may cause subsynchronous oscillation. Static var compensators (SVC) can compensate reactive power to HVDC and mitigate subsynchronous oscillation with additional control. In order to gain accurate control and easily get signal of voltage and current for additional control, instantaneous reactive power theory should be used to eliminate the influence of harmonic component or asymmetric component. Xiangshang HVDC 75% DC power operation model is applied to verify available mitigation of SVC to subsynchronous oscillation.