For　semiconductor-based　photocatalytic　reactions,　defect　engineering　has　been　proven　as　an　efficient　approach　to　enhance　the　photocatalytic　performance.　In　this　work,　a　synergistically　PVP/EG-assisted　in　situ　self-assembly　strategy　has　been　successfully　developed　for　preparing　flowerlike　BiOCl　nanospheres　(NSP)　assembled　by　ultrathin　nanosheets　(thickness　of　3.8　nm)　with　abundant　oxygen　vacancies　(OVs).　During　the　hydrothermal　process,　PVP　plays　a　template　role　in　controlling　the　orientation　of　the　crystallite　growth,　leading　to　the　forming　of　nanosheets.　Meanwhlie,　ethylene　glycol　would　induce　the　self-assembly　of　nanosheets　into　a　loose　hierarchical　architecture　duo　to　its　stereo-hindrance　effect.　NSP　achieves　a　twice　higher　photocatalytic　conversion　of　benzylamine　than　BiOCl　nanosheets　(NST)　under　visible　light.　XPS,　ESR,　NH3-TPD　results　manifest　that　NSP　possesses　more　active　sites　including　OVs　and　unsaturated　Bi　atoms　than　NST,　because　of　avoiding　the　accumulation　of　ultrathin　nanosheets.　In　situ　FTIR　reveals　that　benzylamine　molecules　can　be　chemisorbed　and　activated　on　BiOCl　interfaces　via　forming　-N…Bi-　species.　The　OVs　can　facilitate　the　forming　of　superoxide　radicals　(•O2−),　achieving　the　selective　photooxidation.　Finally,　a　possible　synergetic　mechanism　based　on　the　interaction　of　reactants　and　catalyst　interfaces　was　proposed　to　illustrate　the　photocatalytic　process　at　the　molecular　level.　©　2021　Elsevier　Inc.