The invention discloses an unpowered hydraulic slowly-decreasing dock leveller, which is provided with a base and a bridge plate, wherein the middle part of the bridge plate is hinged with the base by an intermediate hinge pin, the front part of the intermediate hinge pin is provided with a front buffer cylinder, the rear part of the intermediate hinge pin is provided with a rear buffer cylinder,and the two ends of the front buffer cylinder and the rear buffer cylinder are respectively hinged with the bridge plate and the base; the lower cavity of the cylinder body of the front buffer cylinder body and the lower cavity of the cylinder body of the rear buffer cylinder are communicated by a hydraulic hose I, the upper cavity of the cylinder body of the front buffer cylinder body and the upper cavity of the cylinder body of the rear buffer cylinder are communicated by a hydraulic hose II, and a throttle valve is arranged on the hydraulic hose I or the hydraulic hose II. By using the invention, a track-laying vehicle can smoothly be changed into a horizontal situation from an inclined situation in the process of boarding, or changed into the inclined situation from the horizontal situation in the process of debarkation; and the invention is simple in structure and does not require power, and avoids the impact on loaded vehicles.