The stability of the electric power steering system is analyzed on the basis of the mathematics model for pinion -and- rack steering system, the interference resistance is also studied by the way of fixing the steering wheel. The result shows that the stability of the system becomes poor gradually along with the increase of power ratio. The stability can be improved by PD control algorithm, but the influence on the system caused by the measure noise of torque sensor cannot be controlled. If PD combines with the Hinfin optimal control, the stability and interference resistance can be effectively improved.