The invention discloses a subsynchronous oscillation assessment method of a power system including a Unified Plow Flow Controller (UPFC) and belongs to the technical field of power system planning and operating. The method firstly establish a subsynchronous oscillation equivalent research model of the UPFC, then makes proper equivalent simplification of the power system according to a network structure and a generator and obtains an equivalent model from the generator to the UPFC, and finally uses the equivalent model from the generator to the UPFC for assessment and prediction of subsynchronous oscillation behaviors of the power system. The method achieves subsynchronous oscillation assessment of the power system with the UPFC and can be used to guide planning and safe and stable operation of a power grid.