A hydraulic power device is provided. The hydraulic power device includes: a first cylinder with a cylindrical chamber, a first fluid inlet, a first fluid outlet, and a weight unit having a first piston. The first piston hermetically fits into the first cylinder and moves upwards and downwards within the first cylinder. When the first piston moves downwards within the first cylinder, the fluid in the cylindrical chamber flows out through the first fluid outlet under the pressure from the first piston, and thereby generates hydraulic power. When the first piston moves upwards within the first cylinder, the fluid flows into the cylindrical chamber through the first fluid inlet, to get prepared for subsequent hydraulic power output. The hydraulic power device in the present invention is designed to lift a weight unit with a small initial power and then generate hydraulic power utilizing the gravity of the weight unit.