The frequency stability problem brought about by the asynchronous operation of the Southwest Power Grid is outstanding. The research results of the suppression of ultra-low frequency oscillations need to be further practiced in the actual power grid. This paper firstly summarizes the problem of ultra-low frequency oscillation, and points out that the root of ultra-low frequency oscillation lies in the control system of hydropower unit prime mover. Then the limitations of existing oscillation suppression measures are analyzed and the additional damping control method based on governor (GPSS) is proposed. The ultra-low frequency oscillation suppression method is based on the time-frequency analysis method, and the control parameters are set. The single-machine-loaded hydropower unit speed control system model is built in Simulink to verify the suppression effect of GPSS. Finally, the semi-physical simulation test platform is built and the ultra-low frequency oscillation suppression effect of GPSS is verified, which provides practical support for the application of GPSS in the actual power grid.