Ultrathin　Bi12O17Cl2　nanosheets,　with　the　thickness　of　3-4　nm,　were　prepared　by　the　photo-assisted　Ag　exfoliation　method.　This　method　not　only　achieves　the　thickness　regulation,　but　also　introduces　elemental　silver　on　the　surfaces　of　ultrathin　Bi12O17Cl2　nanosheets.　The　obtained　ultrathin　Bi12O17Cl2　nanosheets　modified　with　Ag　show　enhanced　visible　light　absorption,　improved　carrier　generation　and　separation　efficiency　under　visible　light　irradiation.　Their　photocatalytic　performance　for　rhodamine　B　(RhB)　degradation　is　5　times　higher　than　that　of　bulk　Bi12O17Cl2.　It　is　expected　that　the　present　synthetic　strategy　may　provide　a　new　design　concept　for　the　fabrication　of　ultrathin　structures.