Diaphragm type pressure relieving/sustaining valve is installed in feeding water system's pipeline in high building, fire protection and other When the line pressure is beyond the valve's setting pressure, the valve automatically opens to relieve pressure to ensure the pipeline is safe, and at the same time it can be used as pressure sustaining valve to ensure the upstream pressure is enough. The valve can provide and unchanged setting pressure and once the pressure is over, the valve opens quickly and relieve the over pressure. The valve can close stably and smoothly and prevent the pressure repercussions.