The adjustable pressure-reducing valve is a diaphragm-type hydraulically operated type, installed in the water supply and drainage system, and controls the outlet pressure of the main valve to a fixed value. The inlet pressure can be reduced to a required outlet pressure by adjusting the pilot valve and the outlet pressure can be kept stable by relying on the energy of the medium itself, that is, the outlet pressure does not change due to changes in inlet pressure or outlet flow. When the pressure in front of the valve is lower than the set pressure behind the valve, the main valve will automatically close slowly.