After obtaining the optimal trajectory through the lane change decision and trajectory planning, the last key technology for the automatic lane change assist system is to carry out the precise and rapid steering actuation according to the front wheel angle demand. Therefore, an automatic lane change system model including a BLDCM (brushless DC motor) model, a steering system model and a vehicle dynamics model is first established in this paper. Electromagnetic characteristics of the motor, the moment of the inertia and viscous friction etc. are considered in these models. Then, a SMC (Sliding Mode Control) algorithm for the steering system is designed to follow the steering angle input. The control torque of the steering motor is obtained through the system model according to steering angle demand. After that, the control current is calculated considering of electromagnetic characteristics of the BLDCM. Debugging and optimization of the control algorithm are done through simulations. Also, different steering input and uncertain disturbance torque are successively loaded on the simulation model to test the tracking performance and robustness of the control algorithm. Simulation results are compared with the PID’s and show that under different steering inputs, the steering system with the proposed controller manifests a better tracking performance. Even if large parameter uncertainties are taken into consideration, tracking error will not be enlarged obviously, which indicates that the SMC has good robustness. These results can provide reference for precise steering angle control of the automatic lane change assist system by considering of different parts of the steering system, steering motor, steering transmission linkage, and steered wheels.