Pressure control accuracy of hexahedral press affects the quality of synthetic diamond. In order to reduce the pressure fluctuation caused by frequent starting and stopping of threephase asynchronous motors, a pump control pressure compensation system press based on a permanent magnet synchronous motor (PMSM) was designed. The motor driver with vector control was designed, and according to the established mathematical model of the hydraulic system, the designed system was simulated under Simulink environment. The simulation results show that using the system, the pressure compensation precision requirement can be reached, the continuous pressure compensation of the motor at low speed is realized, and the reliability of the pressure control system is effectively improved, production and maintenance costs can be reduced