In　this　paper,　the　design　of　knee　assistive　exoskeleton　mechanism　was　discussed,　which　could　enable　human　knee　axis　to　align　the　exoskeleton　active　joint　axis　ideally.　Based　on　the　data　of　the　knee　joint　flexion　experiment,　the　knee　joint　screw　motion　parameters　could　be　extracted.　According　to　the　knee　joint　physiological　structure　and　the　instantaneous　characteristics　of　screw　axis,　the　knee　joint　kinematic　model　was　established.　Then,　according　to　the　DOFs　analysis　of　human-machine　closed　chain　and　the　human-machine　compatibility,　a　novel　knee　exoskeleton　mechanism　configuration　was　proposed.　And　the　positional　solution　of　human-machine　closed　kinematic　chain　was　solved.　It　can　be　found　that　the　motion　between　knee　assistive　exoskeleton　mechanism＇s　joint　and　human　knee　joint　had　certainly　quantificational　relationship,　and　the　knee　assistive　mechanism　had　a　fine　motion　synchronicity　with　the　human　knee　joint.　In　addition,　while　the　method　of　screw　axis　and　the　exoskeleton　design　were　proposed　for　human　knee　joint,　they　are　also　applied　to　other　human　limbs.