A new structure rotary magnetorheological damper with three working surfaces was designed, which were two end surfaces and one cylindrical surface of the rotor. The area of the working surfaces was increased with the same volume of the rotor. Base on properties of magnetorheological fluid and magnetic core material, the magnetic circuit was designed, and the mechanical and electrical parameters of the damper were calculated. Depending on the characteristics of the magnetorheological damper structure, the finite element model of the magnetorheological damper was built, and the electromagnetic field analysis was performed. Then the magnetic field quantities of the damper model were got. With the results of the magnetic analysis and the constitutive relations, the performance simulation of the damper was conducted. On these bases, the magnetorheological damper and the mechanical performance test system were developed, and the mechanical performance of the damper was investigated using the test system. The damper properties, torque vs rev and yielding torque vs current, were obtained.