Abstract: A new MR damper with bidirectional adjusting damping force is designed to overcome the disadvantages of the traditional magnetorheological (MR) damper, whose damping force is too low in the state of zero current, and the inverted MR damper that is unable to be fully demagnetized. A new structure of composite magnetic circuit is proposed, and the structural and magnetic circuit design of the damper is accomplished through the finite element analysis on the magnetic field. The outputs of the damper in three states of zero, maximum and minimum input currents are predicted according to the Bingham plate model, and the force-velocity curves of the damper in the three states are obtained. It is shown that the proposed composite magnetic circuit may achieve the bidirectional adjustment of damping force, and the distribution of the outputs of the damper in the three states is reasonable and meets the design requirement. The output of the damper in the state of zero current is between the maximum and minimum outputs, which not only ensures the fail-safe property of the damper, but also avoids the locking phenomenon of piston. The settling of the MR fluid may be prevented because a definite magnetic flux is maintained in the damping tunnel of the damper in the state of zero current.