Nowadays, the hydraulic straightening presses play an important role in straightening mandrels. For different mandrels, straightening forces and reductions differ. So the proportional position control system under pressure boundary conditions is required. The mathematical model of the system is deduced, and simulation of the system is carried out by MATLAB. The curves of step response and Bode diagram show that the system is steady and quick-response. The error caused by load is analyzed, which meets the straightening requirements and validates its design. The work in this paper can provide a high guidance for presses of similar kinds.