Diaphragm type adjustable pressure reducing/sustaining valve is installed in feeding/draining water system in high building to reduce the inlet pressure required pressure. The valve ensure outlet pressure stay in the setting valve stably on its own energy, namely the outlet pressure does not change owing to the inlet pressure's and flow's change. There is self-cleaning filtering mesh installed at the valve control system's inlet not to let the great specific gravity and large diameter particle flow in to the system and ensure the system is unimpeded. The valve acts quickly and has a long life