In traditional Active Front Steering System, the ideal variable steering ratio curve has the defect of non-smooth property, thus inducing torque fluctuation of the electric motor. This paper aims at designing a smooth fitting variable steering ratio curve based on vehicle model, through a comparative analysis of five different kinds of fitting variable steering ratio curves. The analysis indicates that the modified sigmoid fitting curve has the best overall performance due to its closeness to the ideal curve and mininal influence on the torque fluctuation of the electric motor. Furthermore, simulation in MATLAB/Simulink validates the good property of the modified sigmoid fitting curve.