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With the development and application of wind power generation and wave power generation, the hydraulic pump-motor control system has a unique effect on power conversion and constant speed driving of power generator.In this situation, the hydraulic pump-motor control system is required that the output speed of the hydraulic motor must be constant when the flow-rate input is randomly varied.Therefore, the power generator driven by the hydraulic motor can maintain constant speed and generate constant frequency electricity power.Considering the particular requirement mentioned above, this paper proposes a stratified-adjustment closed-loop speed control system which is rough-tuned by a variable-displacement motor and fine-tuned by a bypass proportional flow control valve and a control strategy based on dual-PID method.When the flow-rate from the hydraulic pump is randomly varied, the displacement of the variable-displacement motor and the opening of the bypass proportional flow control valve are stratified adjusting by a dual-PID controller, so that the output speed of the variable-displacement motor can maintain constant despite the flow-rate input varying.The simulation and the experiment show that the dual-PID stratified algorithm is effective for constant speed control of hydraulic motor under variable flow-rate input, and it fulfills the requirement of generating constant frequency electricity power.