In order to make better use of effective magnetic field area in the damping channel of long stroke MR damper and control flexibility, to realize the stable control of damping force and displacement, the magnetic field of MR damper was simulated and analyzed. Then the non-parametric model of magnetic induction density distributed in the effective length of damping channel was given when the four coils were applied with different currents. It lays foundation of control algorithm for the control object of impact load.