In view of the power limitation and low speed output instability of BLDCM in EHPS system for medium and heavy vehicles， permanent magnet synchronous motor（PMSM） is used as the power motor of the system in this paper. Based on the structure principle and control strategy of EHPS system， a controller with STM32F103zet6 chip as its core is designed and manufactured. Hardware design includes MCU minimum system circuit， power supply circuit， signal acquisition circuit， power module circuit， CAN communication circuit， fault and high temperature alarm circuit， etc. In terms of control strategy， a vector control method with i_d=0 is used to control sensorless permanent magnet synchronous motor. In the aspect of software implementation， the control decision-making part， motor drive control part， fault diagnosis and transmission part are mainly designed. The design of power steering system mainly includes： resistance torque of in-situ steering， output torque of steering gear， pressure of steering system， working flow of steering pump and power of steering motor. Finally， the bench test and EHPS system test of the controller are carried out， and the control effect of the controller is tested. The control effect of the controller on the motor is tested in the bench test. The results show that the target speed of the motor responds quickly and the output is stable. The controller developed in this paper realizes the control of the permanent magnet synchronous booster motor. EHPS system test has tested the effect of the controller on the passenger car steering assistance. The EHPS system test results show that： when the self-developed EHPS system controller works， the torque of steering wheel control is significantly reduced， and the steering assistance system has good performances. With increasing vehicle speeds， the steering wheel power moment decreases， which realizes the dynamic change of powers， and meets the requirements of the bus conventional power mode.