A factional-order derivative model of the magnetorheological damping system was addressed, an analytical solution and an effective numerical solution of the model were given. The displacements in different currents were adopted here to study the nonlinear characteristics of the damping system. Experiments show that fractional-order derivative model is more precise than integer-order derivative model of the transfer function, and the nonlinear characteristics of the damping system are related to the fractional orders. And the orders are associated with the control currents of the damping system. It is indicated the model of fractional-order derivative is feasible and effective for investigation of magnetorheological vibration device. 