A hydraulic door closer capable of reducing oil-pressure therein at high temperature, comprises a housing with an oil chamber, an oil storage cavity, and a piston in the housing of the hydraulic door closer. The oil storage cavity is in communication with the oil chamber; when the oil temperature increases, hydraulic oil in the oil chamber and oil storage cavity generates an increased oil pressure move the piston, whereby volume of the oil storage cavity is enlarged, such that oil from the chamber will flow into the cavity, to reduce the oil pressure of the chamber and cavity. When the oil temperature decreases, the oil pressure in the chamber and cavity is reduced, whereby the storage piston restores its original position, whereby volume of the cavity will be reduced, and the oil in the cavity flows back into the chamber. The pressure in the oil chamber is reduced when the hydraulic oil expands due to increased temperature to prevent failure of the seal and to avoid hydraulic oil leakage.