Monolayer　H1.07Ti1.73O4　center　dot　H2O　nanosheets　with　the　thickness　about　0.67　nm　were　prepared　and　developed　as　an　efficient　photocatalyst　for　hydrogen　evolution.　The　prepared　sample　exhibits　greatly　improved　photocatalytic　activity　with　more　10.5　times　higher　than　its　layered　counterpart.　The　morphologies,　microstructures,　superficial　properties　and　electronic　structures　of　the　sample　were　characterized　by　XRD,　TEM,　AFM,　BET,　and　UV-vis　DRS　in　detail.　Moreover,　EXAFS,　FTIR,　XPS　and　in-suit　FTIR　of　D2O　absorption　results　suggested　that　Ti　vacancies　result　in　the　formation　of　abundant　active　O　species　around　vacancies　sites,　which　can　be　exposed　fully　in　the　monolayer　nanosheets　and　bind　with　water　molecules　in　the　formation　of　surface　coordination　via　hydrogen　bonds.　An　efficient　electron　transition　from　nanosheets　to　surficial　coordinated　H2O　molecules　takes　place.　Finally,　a　synergistic　effect　between　titanium　vacancies　and　ultrathin　2D　structure　was　proposed　to　elucidate　that　the　enhanced　photocatalytic　performance　over　metal　defects　may　be　attributed　to　efficient　exposure　of　active　species　and　transition　of　photo-electrons　from　surface　to　H2O　molecules.