Abstract: According to the characteristics of forced oscillation,the linearized state-space method is used to analyze the mechanism of the unified power flow controller（UPFC）,which can improve the system damping ratio and suppress forced oscillation.A plug-in modified repetitive control（PMRC）is designed to suppress the forced oscillation.And its tracking performance for the system steady-state reference value proves to be good.However,considering that PMRC cannot output power,a method is proposed to combine the power control device UPFC with PMRC to output the suppression power.The method can make UPFC output the suppression power on the basis of improving the system damping ratio,thus the forced oscillation can be suppressed from multiple aspects.The simulation shows that the proposed method can effectively suppress the forced oscillation of the power system.