Backflow prevention device works by using medium water flow through the valve due to mechanical resistance and head loss through water decompression, so that inlet pressure is higher than the pressure vent valve cavity and the upper vent valve diaphragm pressure inlet pressure, the pressure is greater than the upper vent valve diaphragm chamber and the spring resistance, the drain valve to maintain close, the normal flow of medium, when the import pressure and the pressure vent valve cavity is less than 0.02Mpa, the media in the back of the critical state , then the pressure is less than the upper vent valve Diaphragm vent valve cavity and the spring pressure, vent valve automatically opens, the discharge pipe outside the media back, while a large number of air into the valve chamber, the formation of air cut off, to prevent the media back, large when the drain valve at 0.02Mpa automatically shut down.