To improve damping force and reduce response time of magnetorheological (MR) damper, an optimal structure designing method was presented. The structure optimal methods, including reverse series of excitation coils and composed permanent magnets piston structure, was based on the calculation of resultant magnetic field for different excitation coil winding approachs. According to the provided structure optimal method, a MR damper was manufactured and its mechanical performances were tested on type DS-100 fatigue-testing machine with different forcing vibration frequency, vibration amplitude and control current. The experiments showed that the damping force of the proposed MR increased by 65%, and the response time reduces to more than a half at the most. This result provided an important reference to further mechanical property improvement of MR damper.