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.