The　self-aligning　capability　of　an　exoskeleton　is　important　to　ensure　wearing　comfort,　and　the　delicate　motion　ability　of　the　exoskeleton　is　essential　for　motion　assistance.　Designing　a　self-aligning　exoskeleton　that　offers　improved　wearing　comfort　and　enhanced　motion-assistance　functions　remains　a　challenge.　This　paper　proposes　a　novel　spatial　self-aligning　mechanism　for　a　knee　exoskeleton　to　enable　simultaneous　assistance　in　the　flexion　and　extension　(FE)　of　the　knee　joint　and　the　internal　and　external　rotation　(IER)　of　the　hip　joint.　Additionally,　considering　the　misalignment　of　the　human-robot　joint　axes,　a　kinematic　model　of　the　knee　exoskeleton　is　established　and　analyzed　to　demonstrate　the　kinematic　compatibility　of　the　exoskeleton.　Furthermore,　a　global　torque　manipulability　(GTM)　index　is　proposed　to　evaluate　the　effects　of　dimensional　parameters　on　the　exoskeleton＇s　performance,　and　then　the　knee　exoskeleton　is　optimized　according　to　the　GTM　index.　Finally,　experiments　are　conducted　to　validate　the　performance　of　the　proposed　exoskeleton.　The　experimental　results　show　that　during　knee　FE　and　hip　IER,　the　proposed　exoskeleton　exhibits　lower　interaction　forces　and　torques　than　existing　exoskeletons.