Based on Bingham model，the equation of Magnetorlogical fluid through a slot valve is derive. The relationship between the damper force ，the structure parameters of MR damper and the performance parameters of MR fluid is shown as a quartic algebraic equation. The equation is solved and effect of the MR viscosity，yield stress and the structure parameters of MRFD on damper performance is analysed.The research in this paper can instruct MR damper design.