The utility model discloses an overflow valve, include upper cover and the valve body that is provided with the valve port, still be provided with air inlet and gas outlet on the valve body, the embedded piston that is equipped with of valve body, the spring is equipped with on piston upper portion, forms the pressure chamber between piston and valve body, and this pressure chamber communicates with each other with the air inlet to through gas outlet intercommunication atmosphere, the piston is provided with the vulcanite pad that is used for sealed valve port with the corresponding departmentof valve port. The utility model discloses an accuracy of overflow valve operating pressure is high, the operational reliability is high, longe -lived, can be used to the locomotive high to the required precision.