Bi5O7I　photocatalysts　are　considered　potentially　excellent　marine　antifouling　materials.　However,　the　photo　catalytic　properties　differ　for　Bi5O7I　photocatalysts　with　various　morphologies,　and　the　photocatalytic　mechanisms　involved　remain　unclear.　In　addition,　powdered　Bi5O7I　photocatalysts　are　difficult　to　work　with　in　an　exact　manner　on　marine　structure　surfaces,　such　as　concretes,　in　actual　marine　environments.　Thus,　in　this　study,　Bi5O7I　photocatalysts　with　various　morphologies　comprising　nanosheets,　nanorods,　nanoflowers,　and　micro　spheres　(MS)　were　successfully　synthesized.　Among　these　Bi5O7I　photocatalysts,　the　highly　regular　Bi5O7I　MS　(Bi5O7I-MS)　were　found　to　have　the　highest　photocatalytic　properties　in　both　the　degradation　of　Rhodamine　B　and　inactivation　of　bacteria.　This　strong　photocatalytic　ability　can　be　attributed　to　the　high　specific　surface　area　and　rich　oxygen　vacancies　(OVs)　in　Bi5O7I-MS.　The　possible　photocatalytic　mechanism　involves　h+　playing　the　major　role,　with　&　BULL;OH　and　&　BULL;O2　　partially　contributing　to　the　photocatalytic　process.　OVs　also　promoted　the　photocatalytic　properties　of　Bi5O7I-MS　by　trapping　photoinduced　electrons　to　accelerate　the　photocatalytic　process.　Furthermore,　Bi5O7I-MS　photocatalysts　were　successfully　grown　in　situ　on　cement　mortar　surfaces,　where　they　showed　extremely　high　antifouling　performance,　with　hardly　any　living　bacteria　attached.　These　findings　clarify　the　morphology-dependent　mechanism　associated　with　Bi5O7I　photocatalysts,　as　well　as　facilitating　the　growth　of　photocatalysts　in　situ　on　cement　mortar　for　applications　in　marine　antifouling.