The　O-2　preadsorption　properties　prior　to　the　application　for　nanomaterials　have　rarely　attracted　attention;　however,　they　greatly　affect　the　surface　nature　between　gas　and　nanomaterials.　Here,　a　hierarchically　ZnO　nest-like　architecture　(ZnO　NAs)　with　nanosheets　was　synthesized　by　a　facile　hydrothermal　method　without　structure-directing　agents　and　templates.　The　percentage　of　exposed　(001)　facet　for　ZnO　NAs　is　similar　to　95%　according　to　its　micromorphology.　A　gas　sensor　fabricated　by　ZnO　NAs　exhibits　high　sensitivity,　low　detection　limit,　fast　response,　and　good　selectivity　to　acetone　at　the　low　working　temperature　(105　degrees　C).　The　distinct　gas-sensing　properties　of　ZnO　NAs　are　mainly　attributed　to　the　specific　surface　area　(63.46　m(2)/g)　and　high　active　(001)　facet　for　the　nanosheets.　Note　that　a　preadsorption　of　O-2　from　air　on　ZnO　NAs　and　the　gas　reaction　mechanism　are　put　forward　based　on　the　preadsorbed　behavior　and　target　gas　response.　Moreover,　by　the　aid　of　first-principles　on　the　analysis　of　its　surface　adsorption　energy　and　adsorption　structure　at　(001)　facet　of　ZnO　NAs,　it　is　identified　that　an　oxygen　preadsorption　step　on　the　facet　occurs　once　it　makes　contact　with　air　due　to　a　lowest　surface　adsorption　energy　(-3.149　eV)　for　oxygen　molecule.　After　the　O-2　preadsorption　onto　the　surface,　acetone　is　with　the　lowest　surface　adsorption　energy　of　-0.687　eV,　assigned　to　a　chemical　adsorption　compared　with　the　other　gases.　It　benefits　the　acetone　adsorption　on　the　(001)　facet　for　ZnO　NAs,　as　well　as　following　electron　transfer　and　gas　response.　The　sensitivity　and　selectivity　for　gas　sensor　based　on　ZnO　NAs　are　well　certified　by　both　gas-resistance　response　and　computational　simulation.