the present invention relates to an air pressure regulator , characterized in that a regulator made of a piston body connected with a return spring is disposed at the top of a main body , wherein the high - pressure gas flowing from the primary pressure chamber to the secondary pressure chamber can be controlled by means that the secondary pressure inside of the closed space created by the piston body and the top of the main body exerts a stable and balanced effect on the piston body which raises or lowers the plug assembly , and that a stable output and a high accuracy of the secondary pressure can be ensured by means that the resilience of the return spring and the gas pressure creates a coordination effect without being influenced by the pressure difference created by the change of the internal pressure inside steel cylinder after bottling , so that the using life thereof can be extended and its safety can also be enhanced .