The utility model relates to an improved non-return valve mounted on a cold and hot water mixing valve. A cold and hot water inlet in the bottom surface of a box is provided with a non-return valve pit chamber, and the pit chamber is provided with a first stage labial margin and a second stage labial margin. The non-return valve is provided with a casing of a valve, which is just inserted in the pit chamber, and the casing of the valve is provided with a locating ring. The locating ring of the non-return valve is supported on the second stage labial margin of the pit chamber. The casing of the valve on the lower end of the non-return valve and the first stage labial margin together form a pit, the cross section of which presents a U shape. A sealing ring is embedded in the pit to prevent the leakage among the assemblies. The non-return valve can reach the effect of easy and fast assembly and operation.