Structure and application: This kind of valve makes up of main valve, pilot valve, pin valve, ball valve and pressure meter etc. The pressure releasing/sustaining valve can effectively release the pressure that exceeds the safety set point of pilot valve in the pipeline to prevent the pipeline and equipments from damage. This valve is mainly used for the pressure escaping of the circulating system of fire control testing in the high buildings to avoid the system trouble because of highest hydraulic pressure, and can keep the water supply pressure of the main valve always above one set value so as to ensure the pressure of above main valve water supply area.