With the rapid development of DC asynchronous interconnection of power grids, a large number of intermittent renewable energy sources are connected and the load fluctuation is reinforced. In order to ensure the frequency quality of the power grid, higher requirements are imposed on the primary frequency regulation performance of the hydropower units. At present, the governor of hydropower units can meet the primary frequency regulation requirement of the power grid by adjusting the PID parameters, but these para-meters after being adjusted may affect the dynamic stability of the system and cause the system to experience ultra-low frequency oscillation. In order to solve the above problems, this paper proposes a PID parameter switching method based on Prony analysis for the hydropower unit governor. Different control parameters are used to realize sectional control, so that the governor can simultaneously take into account the primary frequency regulation performance and the dynamic stability damping level, thus realize suppression of ultra-low frequency oscillation of the power grid from the power plant side. The effectiveness and robustness of the proposed method are verified by the time domain simulations of the actual system of Yunnan Power Grid after asynchronous interconnection.