Abstract: The large difference in response speed between the governor and the converter of the full-power variable-speed pumped storage unit (full-power VSPS) under power priority control brings about the potential risks of speed undershoot, oscillation and even instability. Therefore, a linearized model of the full-power VSPS under power priority control is established, and the key influencing factors of the unit speed stability are extracted. Accordingly, an analytical expression of the speed stability domain with the governor control parameters as the independent variable during the unit output regulation is derived. Compared with the conventional stability domain calculation method, this expression greatly reduces the amount of calculation while ensuring the accuracy. Based on the proposed stability criterion, the influence of the unit working head on its stability domain and the influence of the governor parameters on the unit speed characteristics are analyzed, and a simple method for real-time calculation of the optimal value of the governor control parameters is proposed. Simulation results show that this method can be adopted to effectively suppress the speed oscillation phenomenon during the output regulation of the full-power VSPS.