Here　we　report　growth　of　＜　0001　＞-oriented　ultrathin-walled　ZnO　single-crystal　microtubes　with　diameter　of　75-250　mu　m　and　facet　wall　of　＜　500　nm　in　thickness　by　optimized　optical　vapor　supersaturated　precipitation　(OVSP).　The　mechanism　of　ultrathin-walled　microtube　formation　during　OVSP　is　revealed.　The　presintering　temperature　of　precursor　rod　in　the　range　of　600-800　degrees　C　is　found　to　be　critical　to　achieve　the　complete　hexagonal　cross-sectional　geometry　of　microtube.　The　facet　wall　is　then　thinned　down　to　similar　to　450　nm　with　the　temperature　holding　time　increasing　during　OVSP　under　the　lamp　power　of　60%@6000　W.　The　ultraviolet　photoluminescence　indicates　the　exciton-exciton　collisions　(i.e.　p-band)　boosted　in　the　ultrathin-walled　ZnO　microtube.　Considering　the　time-consuming　process　in　presintering　of　precursor　rod　in　the　Molysili　furnace,　the　in-situ　optical　vapor　supersaturated　precipitation　(IOVSP)　is　developed,　in　which　the　optical　presintering　is　first　performed　in　the　image　furnace　using　the　lamp　power　of　30%@6000　W　for　6　h　and　then　the　lamp　power　is　directly　increased　to　60%@6000　W　for　microtube　growth　by　OVSP.　The　cooling　process　and　transfer　of　precursor　rod　from　the　Molysili　furnace　to　the　image　furnace　are　eliminated　and　the　growth　time　can　therefore　be　saved　56.7%.　The　finished　microtube　demonstrates　a　perfect　hexagonal　cross　section　with　smooth　surface　and　thin　facet　wall　of　similar　to　500　nm　enhancing　exciton-exciton　collisions.　The　present　work　provides　a　time-saving　in-situ　method　to　grow　high-quality　ultrathin-walled　ZnO　single-crystal　microtubes　served　as　optical　microcavities　in　the　future　for　the　applications　in　micro/nanophotonics.