30 MN automatic straightening machine is a fully automatic hydraulic straightening machine and the pressure is the largest in China. The hydraulic system of main loop was designed based on proportional directional valve and cartridge valve. The mathematical model of proportional position control system was deduced, and then the graph of time domain and frequency domain was plotted by the toolbox of MATLAB and rectified by PID controller to improve the system trait. The result of error based on single interfere import\|load capacity was analyzed,which could meet the requirements and validates the design. The work provides guidance for the debugging later.