The invention relates the technical field of vehicle power steering, in particular to an electric power steering system. The electric power steering system comprises a vehicle velocity sensor, a torque sensor, a rotation angle sensor, an ignition signal generator, a lateral acceleration sensor and a yaw velocity sensor, wherein the vehicle velocity sensor is used for collecting a vehicle velocity signal; the torque sensor is used for collecting a torque signal of a steering wheel; the rotation angle sensor is used for collecting a rotation angle signal and a rotation speed signal of the steering wheel; the ignition signal generator is used for collecting an ignition signal; the lateral acceleration sensor is used for collecting a lateral acceleration signal; the yaw velocity sensor is used for collecting a vehicle yaw velocity signal; an ECU (Electronic Control Unit) receives signals, processes the signals and outputs a current signal to a motor; and a power torque generated by the motor is added to a hand torque, acting upon the steering wheel, of a driver after being increased and the wheel is driven by a mechanical steering system to steer and return. Through collecting the lateral acceleration signal and the vehicle yaw velocity signal, the active returning function of the vehicle at high and low velocities is improved; therefore, the safety and comfort of the vehicle are improved, the operation stability and safety of the vehicle under extreme working conditions of road surfaces with small slip friction coefficients, large-angle sudden turning and the like are enhanced.