Hydraulic control valves can be divided into diaphragm type and piston type according to their structure. The working principle is the same. They are driven by the pressure difference (P) between upstream and downstream. The hydraulic differential operation of the diaphragm (piston) is controlled by the pilot valve, which is fully adjusted by the hydraulic force, so that the main valve disc is fully opened or closed or in the state of regulation. When the pressure water in the control chamber above the diaphragm (piston) is discharged into the atmospheric or downstream low pressure zone, the pressure value acting on the bottom of the disc and under the diaphragm is greater than the pressure value above, so the main valve disc is pushed to the fully open position; when the pressure water in the control chamber above the diaphragm (piston) cannot be discharged into the atmospheric or downstream low pressure zone, it acts on the diaphragm (piston). ) The upper pressure value is greater than the lower pressure value, so the main valve disc will be pressed to the fully closed position; when the pressure value in the control chamber above the diaphragm (piston) is in the middle of the inlet pressure and the outlet pressure, the main valve disc will be in the regulating state, whose regulating position depends on the joint control role of needle valve and adjustable guide valve in the conduit system. The adjustable pilot valve can open or close its own small orifice through the downstream outlet pressure and change with it, so as to change the pressure value of the control chamber above the diaphragm (piston) and control the regulating position of the square valve disc.