Decompression valve is a kind of water supply system which is used for parallel connection of fire water and domestic water. It is used to allocate water supply direction. When fire occurs, fire fighting urgently needs a large amount of water. It cuts off domestic water immediately to ensure sufficient water for fire fighting. When the pressure of fire water stops decreasing, the valve opens automatically and resumes domestic water supply. This valve makes the system need not set up a special one. The separate water supply network for door fire control greatly saves construction cost and water consumption. The valve has high control sensitivity, safety and reliability, simple debugging and long service life.