The invention discloses a magneto-rheological damper which damping force can be adjusted in two ways, wherein a cylinder is connected with an upper and a lower end covers, a piston rod is inserted in the steel cylinder through a sealing ring which is arranged on the end cover, a non-corrosive steel damping magnetic slab is mounted on two ends of a piston slab, a plurality of the piston slabs are provided and alternately sleeved on the piston rod with a magnetic core sleeve barrel, the end surface of each piston slab is summarily opened with two grooves along the axial direction, a permanent magnetic ring is inserted in the groove, and the outer lateral of a magnetic core sleeve barrel is winded with an excitation coil. The damper respectively realizes the largest output and the smallest output through adding with the forward current and the inverse current, not only guarantees the fail-safe (fail-safe) performance of the damper, but also effectively prevents the occurrence of locking phenomenon, on the other hand, the invention provides a novel design of a composite magnetic circuit, greatly simplifies the composite magnetic circuit structure when the permanent magnet and the excitation coil coexist, which is beneficial to the practical manufacture and installation of the damper.