Due　to　the　close　physical　interaction　between　human　and　machine　in　process　of　gait　training,　lower　limb　exoskeletons　should　be　safe,　comfortable　and　able　to　smoothly　transfer　desired　driving　force/moments　to　the　patients.　Correlatively,　in　kinematics　the　exoskeletons　are　required　to　be　compatible　with　human　lower　limbs　and　thereby　to　avoid　the　uncontrollable　interactional　loads　at　the　human-machine　interfaces.　Such　requirement　makes　the　structure　design　of　exoskeletons　very　difficult　because　the　human-machine　closed　chains　are　complicated.　In　addition,　both　the　axis　misalignments　and　the　kinematic　character　difference　between　the　exoskeleton　and　human　joints　should　be　taken　into　account.　By　analyzing　the　DOF(degree　of　freedom)　of　the　whole　human-machine　closed　chain,　the　human-machine　kinematic　incompatibility　of　lower　limb　exoskeletons　is　studied.　An　effective　method　for　the　structure　design　of　lower　limb　exoskeletons,　which　are　kinematically　compatible　with　human　lower　limb,　is　proposed.　Applying　this　method,　the　structure　synthesis　of　the　lower　limb　exoskeletons　containing　only　one-DOF　revolute　and　prismatic　joints　is　investigated;　the　feasible　basic　structures　of　exoskeletons　are　developed　and　classified　into　three　different　categories.　With　the　consideration　of　quasi-anthropopathic　feature,　structural　simplicity　and　wearable　comfort　of　lower　limb　exoskeletons,　a　joint　replacement　and　structure　comparison　based　approach　to　select　the　ideal　structures　of　lower　limb　exoskeletons　is　proposed,　by　which　three　optimal　exoskeleton　structures　are　obtained.　This　paper　indicates　that　the　human-machine　closed　chain　formed　by　the　exoskeleton　and　human　lower　limb　should　be　an　even-constrained　kinematic　system　in　order　to　avoid　the　uncontrollable　human-machine　interactional　loads.　The　presented　method　for　the　structure　design　of　lower　limb　exoskeletons　is　universal　and　simple,　and　hence　can　be　applied　to　other　kinds　of　wearable　exoskeletons.