The speed regulating valve is a combination valve composed of a fixed differential pressure reducing valve and a throttle valve connected in series. The throttle valve is used to adjust the passing flow, and the constant difference pressure reducing valve automatically compensates for the impact of load changes, so that the pressure difference before and after the throttle valve is a constant value, eliminating the impact of load changes on the flow rate. The pressure in front and behind the throttle valve is directed to the right and left ends of the pressure reducing valve spool respectively. When the load pressure increases, the hydraulic pressure acting on the left end of the pressure reducing valve spool increases, and the valve spool moves to the right, and the pressure reducing port As the pressure increases, the pressure drop decreases, so that the pressure difference (p2-p3) of the throttle valve remains unchanged;and vice versa. In this way, the flow rate of the speed control valve is constant (not affected by the load). The speed regulating valve can also be designed to throttle first and then reduce pressure.