The invention relates to the technical field of on-board power generation, and specifically discloses a hydraulic on-board generator energy-saving power generation system, comprising an engine, a coupling, a quantitative pump, a high-pressure oil circuit, a variable motor, a generator, a low-pressure oil circuit, and a rotational speed torque sensor , displacement sensor, calculation controller and throttle controller, the engine is coaxially connected to the quantitative pump through the coupling, the oil suction port of the quantitative pump is connected to the low-pressure oil circuit, and the pressure oil port of the quantitative pump is connected to the variable motor through the high-pressure oil circuit. The oil discharge port of the variable motor is connected to the low-pressure oil circuit, the variable motor is coaxially connected to the generator, the speed and torque sensor is set between the variable motor and the generator, the displacement sensor is set on the throttle controller of the engine, and the calculation controller is respectively Connected to displacement sensor, rotational speed torque sensor, throttle controller and variable motor. The invention can make the output power of the engine match the power obtained by the generator, avoid power waste, and effectively improve the efficiency of the unit.