Magnetorheological (MR) fluid is capable of changing its rheological characteristics under magnetic field. Controllability is one of the advantages of MR damper. Up to now, the dynamic characteristics of MR damper under impact loads have not been well known by researchers. In this paper, a new type of dynamic model for MR damper subject to impact load is derived. And in this novel model, it is pointed out that the square damping item and the inertia force item are two important factors which influence the total damping force in impact condition. The adding two items make the model more exactly describing its dynamic characteristics of MR damper in impact condition. This action makes the established model easier to do control analysis. Because of the polynomial form and a fixed coefficient, it is indicated that the optimization control method is a good choice for MR damper to track the demanded impact curves. Simulation analysis results prove that based on its controllability of damping force MR damper provide, its vibration-reduction and shock-resistant properties of MR damper under impact load will also be greatly improved.