Lower　extremity　paralysis　has　become　common　in　recent　years,　and　robots　have　been　developed　to　help　patients　recover　from　it.　This　paper　presents　such　a　robotic　system　that　allows　for　two　working　modes,　the　robot-active　mode　and　human-active　mode.　The　robot　is　designed　to　be　equipped　with　magnetorheological　(MR)　actuators　that　have　the　advantages　of　high　torque,　fast　response,　flexible　controllability,　low　power　consumption　and　safety　guarantee.　The　design　and　characteristics　of　the　MR　actuator　are　introduced.　In　the　robot-active　mode,　the　MR　actuator　works　as　a　clutch　to　transfer　the　torque　to　the　robotic　joint　safely.　In　the　human-active　mode,　the　MR　actuator　functions　as　a　brake　to　provide　resistance　to　help　strengthen　muscles.　The　working　mode　is　determined　by　the　human　motion　intention,　which　is　detected　via　the　skin　surface　electromyography　(EMG)　signals.　The　human-robot　interaction　torques　are　estimated　using　the　EMG-driven　impedance　model.　The　biomechanical　analysis　based　on　Any　Body　Modeling　System　(AMS)　is　used　to　help　optimization.　Then,　an　adaptive　control　method　is　proposed　to　realize　the　assist-as-needed　(AAN)　training　strategy,　where　the　robot　can　switch　between　these　two　modes.　Experiments　are　conducted　to　validate　the　proposed　design.