An improved Bingham model was proposed to describe the force of a MR damper, whose primary resonance for the singledegreeoffreedom (SDOF) was researched under the condition that the spring deformation was comparatively great. The analytical solution of the system was obtained by using the Averaging Method, whose effectiveness was verified through numerical simulation, and the effects of all the physical parameters on the amplitudefrequency curves were also studied so as to control the primary resonance effectively. In addition, the amplitude comparison between the traditional damper and the MR damper in the primary resonance was presented, which showed the performance of MR damper in vibration control was better.