Combined with theoretical analysis and the experimental data, a nonlinear sinusoidal viscous loop damping force model on magnetorheological fluid (MRF) was presented. The method for determining the parameters of the model was given. This model could be reflected of the MRF hysteretic behavior very well, and had very good fitting precision of experimental data of selfdesigned damper. The force model is not only good at calculating and controlling, but also has high practical value.