Abstract: To solve the problems of instability and slow speed of ultra-high pressure relief in the synthesis process of cubic press, the pressure relief mechanism is designed and optimized automatically and linearly. Based on the hydraulic control theory, the working model, the control model and the linear pressure relief model of the cubic press are established by using AMESim software. Four kinds of different pressure relief process curves are compiled. Based on the classical PID(proportion integral differential) control theory, the linear pressure relief model is simulated and calculated. Taking the half cone angle of the valve core and the diameter of the throttle orifice of the valve seat as variables, the linear pressure relief scheme meeting the process requirements is obtained. The actual test of the designed linear pressure relief mechanism shows that the absolute value of the pressure deviation between the actual pressure curve and the set pressure curve is less than or eaqual to 0.01 MPa, which meets the expected design requirements.