Previous　bismuth-based　photocatalysts　usually　employ　a　strong　acid　solution　(e.g.,　HNO3　solution)　to　obtain　an　ultrathin　structure　toward　high　photocatalytic　activity.　In　this　work,　the　ultrathin　layered　BiOIO3　nanosheets　are　successfully　synthesized　using　just　the　glucose　hydrothermal　solution.　The　high-concentration　glucose　solution　shows　the　obvious　acidity　after　the　hydrothermal　process,　which　leads　to　the　quick　decrease　in　thickness　of　BiOIO3　nanosheets　from　∼45.58　to　∼5.74　nm.　The　ultrathin　structure　can　greatly　improve　charge　carriers’　separation　and　transfer　efficiency.　The　generation　of　reductive　iodide　ions　brings　about　oxygen　vacancies　in　the　ultrathin　nanosheets,　then　the　defect　energy　level　is　formed,　causing　the　decreased　band　gap　and　improving　the　visible　light　absorption.　Compared　to　thick　BiOIO3　nanosheet　with　little　oxygen　vacancies,　much　higher　carrier　separation　efficiency　and　visible　light　absorption　are　achieved　in　the　ultrathin　nanosheets　with　oxygen　vacancies,　resulting　in　an　excellent　photocatalytic　performance　(0.1980　min-1　for　RhB　degradation),　which　is　much　higher　than　most　other　bismuth-based　photocatalysts.　The　superoxide　radicals　(•O2-)　and　holes　(h+)　are　the　major　active　species　responsible　for　high　photocatalytic　activity.　This　work　affords　an　environmentally　friendly　strategy　to　synthesize　ultrathin　photocatalysts　with　superior　photocatalytic　properties.　©　2024　American　Chemical　Society