Magnetorheological (MR) damper was often used in the semiactive suspension system to attenuate the unwanted vibrations. However, it is important to establish a more accurate mechanical model for MR damper when it was applied in this situation. At present, Bingham model, Sigmoid model, doubleSigmoid model and generalized Sigmoid model are the commonly used mechanical models for MR dampers. However, these models can not describe the phenomena that existed in the MR dampers, such as, the inertial effect, the shearthinning phenomenon and the influence of the accumulator stiffness. Based on this, a new mechanical model, i.e., the adjustable Sigmoid model was proposed. Meanwhile, the simulation results and experimental results of phenomenological model were also conducted and compared. The simulation results demonstrate that the adjustable Sigmoid model can effectively describe the behavior of MR damper. It can embody perfectly the inertial effect and shearthinning phenomenon of MR damper. When this model describes exactly the nonlinear hysteretic characteristics in the lowspeed area and highspeed area, the applied physical concept becomes clearer. In a word, this adjustable Sigmoid mechanical model has a stronger applicability and adaptability as compared to the existed models.