MR　(Magnetorheological)　fluid　is　a　kind　of　intelligent　materials.　And　it　is　used　to　make　MR　damper,　a　kind　of　semi-active　control　device.　A　new　type　of　MR　damper　is　designed　that　has　reverse　effect.　The　design　method　of　the　reverse-effect　damper　is　given　and　a　full-scale　valve　mode　MR　damper　is　designed.　By　the　test,　it　is　proved　that　the　damping　force　is　controllable　and　when　the　electrical　source　is　of　no　effect,　the　damper　can　supply　large　damping　force.　This　semi-active　control　equipment　has　more　security　during　earthquakes.　The　reverse-effect　damper　is　reliable,　practicable　and　economical.　Finally,　several　improved　methods　are　put　forward　to　increase　the　adjustable　sphere　of　the　damping　force.