Based on the project requirements of “50 MN large hydraulic machine's steady pressure relief” and the disadvantages of low precision in traditional hydraulic machine's pressure relief with the large power losses in pressure relief of opening pump, a drive control unit was developed combining with the existing“Drive Unit”patented technology. This unit could push the auxiliary cylinder piston rod to produce the displacement through step motor controlled by PLC lead screw nut mechanism connected with reducer and realized the closed-loop control system using a pressure sensor to feedback signal. It was used to connect the auxiliary cylinder with the main cylinder in parallel, close the main oil line to form a closed volume between the two cylinders and change the volume of the auxiliary cylinder to control the step motor speed accurately. It also realized the main oil cylinder with 0.0033 MPa·s-1 to conduct the pressure relief continuously and smoothly when the pump was close.