Photo-chemical　conversion　of　CO2　into　solar　fuels　by　photocatalysts　has　attracted　significant　attention.　However,　poor　reaction　efficiency　remains　a　huge　obstacle.　Deep　insight　into　the　reaction　mechanism　of　CO2,　especially　the　active　site　of　photocatalyst　could　provide　scientific　basis　for　the　development　of　more　efficient　photocatalyst.　The　high　inertness　of　CO2　and　the　multi-electron　reduction　feature　on　a　photocatalyst　determine　high　complexity　of　the　reaction　for　the　study.　Here,　pure　Bismuth　oxyhalides　(BiOX,　where　X　=　F,　CI,　Br,　I)　with　the　layered　structure,　which　were　synthesized　by　both　hydrothermal　method　and　chemical　precipitation　method,　were　selected　as　model　photocatalysts.　The　photocatalytic　behaviors　of　the　samples　were　evaluated　by　the　CO2　reduction　with　H2O　without　the　additional　photosensitizer　and　sacrificial　agent.　The　as-prepared　BiOBr　was　observed　to　exhibit　the　best　CO2　photoreduction　performance　under　the　simulated　sunlight.　The　evolution　rates　of　CO　and　CH4　are　21.6　μmol　g−1　h−1　and　1.2　μmol　g−1　h−1,　respectively.　The　effects　of　water　dosage,　light　intensity　and　irradiation　time　on　the　efficiency　of　CO2　photoreduction　were　investigated　systematically.　Interestingly,　the　reduction　selectivity　of　CO2　to　CO　almost　reaches　100%　in　the　case　of　high　light　intensity.　By　combination　with　isotopic　tracing　method,　electron　spin-paramagnetic　resonance　(ESR),　in-situ　Fourier　transform　infrared　(FTIR)　characterization,　positron　annihilation　lifetime　(PAL)　spectra,　and　Density　functional　theory　(DFT)　calculation,　the　oxygen　vacancy　mediated　mechanism　of　photoreduction　CO2　was　suggested　for　BiOX.　This　work　provides　new　information　and　insights　to　deepen　the　understanding　for　defect　photocatalysis　on　CO2　reduction　of　semiconductor.　©　2020　Elsevier　B.V.