In order to overcome the shortage of the previous model of EPS, the dynamic model for steering system in steering and aligning condition was established, based on the combination of tire model of parking steer and 7 degree-of-freedom vehicle model with Doguff tyre model. For obtaining the characteristic of vehicle steering moment and return ability, the steer torque and return ability of manual steering system was analyzed at all operating conditions. The simulation result and road test showed the model built can accurately describe steering torque and return ability well and truly. Finally, a control strategy for controlling power assistant and self-align was framed based on sliding mode control theory (SMC). The simulation result and road test showed the sliding mode control strategy based on the model improved the steering easiness and vehicle self-align capability effectively. 