The loading rate dependency of the hysteretic dynamics of Magnetorheological (MR) dampers is investigated in the current paper. The model is constructed on the basis of a phenomenological phase-transition theory. The hysteretic dynamics is treated as the responses of a nonlinear system upon external loadings. With appropriately chosen coefficients, the proposed model is able to capture the loading-rate dependency feature of the hysteretic dynamics. Comparisons between the numerical and experimental results are presented, and perfect agreements are obtained.