Magnetorheological (MR) dampers with fixed damping gaps and short damping lengths have the disadvantages of small damping force, single dynamic performance and low adaptability. To overcome these shortcomings, a MR damper with adjustable damping gaps was proposed and designed. The MR damper had annular and conical fluid flow channels, and four segments of effective damping lengths in the annular and conical fluid flow channels. By adjusting the position of the valve core controlled by the locking nut, the relative positions between the valve core and the left and right yokes would be changed, so the damping gaps in the conical fluid flow channel can be adjusted continuously. The magnetic circuit of the proposed MR damper was studied and its mechanical model was also established. ANSYS was used to simulate the change of the electromagnetic field in the MR damper. A prototype was manufactured, and its dynamic performance was tested experimentally. The results showed that the damping force of the proposed MR damper was increased with the decrease of the damping gaps, and the maximum damping force can reach 7.2kN at the damping gap of 0.6mm. Moreover, the adjustable coefficient was decreased with the decrease of damping gaps, and the maximum adjustable coefficient can reach 13.6 at the damping gap of 1.6mm. In addition, the damping force was ranged from 0.2kN to 7.2kN with different damping gaps, and the maximum adjustable coefficient can reach 33. The dynamic performance of the developed MR damper can be greatly enhanced, and the traditional MR dampers with its application in fixed situations can be possibly replaced.