Sunlight-driven　photocatalytic　water　splitting　to　generate　oxygen　(O-2)　is　a　promising　approach　for　utilizing　solar　energy.　Herein,　direct　Z-scheme　heterostructure　photocatalysts　composed　of　ultrathin　Bi3O4Cl　and　BiOCl　nanosheets　are　rationally　fabricated　via　alkaline　chemical　etching　and　solvent　exfoliation　for　O-2　evolution　under　visible　light.　With　AgNO3　and　FeCl3　as　the　electron　scavenger,　the　optimized　ultrathin　Bi3O4Cl/BiOCl　exhibits　prominent　photocatalytic　activity　for　O-2　production　under　visible-light　illumination　and　the　production　rate　(Fe3+:　58.6　mu　molg(-1)h(-1))　is　much　higher　than　the　nanocrystal　heterostructure　(Fe3+:　28.5　mu　molg(-1)h(-1)).　This　ultrathin　heterostructure　system　can　efficiently　transfer　the　electrons,　which　leads　to　a　considerable　improvement　in　the　photocatalytic　performance.　Due　to　the　suitable　band　edge　potentials　and　the　intense　electronic　interaction　between　two-dimensional　(2D)　Bi3O4Cl　and　2D　BiOCl,　as　confirmed　by　theoretical　computations,　photoluminescence,　and　photoelectricity　tests,　the　ultrathin　heterojunction　with　an　internal　electric　field　has　a　highly　remarkable　charge　transfer.　The　intimate　interface　contact　and　{0　0　1}　facets　effect　promote　the　high　photocatalytic　performance　of　the　ultrathin　Bi3O4Cl/BiOCl　heterostructure.