Red　phosphorus　(RP)　has　been　demonstrated　for　photocatalytic　hydrogen　evolution　(PHE)　in　recent　years.　Realistically,　P　vacancy　(VP)　defects　are　present　in　RP.　The　constructive　or　detrimental　influences　of　such　defects　on　the　photocatalytic　activity　of　RP　should　be　revealed　as　the　understanding　shall　provide　a　platform　for　further　improvement.　Herein,　we　for　the　first　time　establish　a　comprehensive　understanding　of　the　crucial　relationships　between　the　intrinsic　VP　defects　and　the　charge　dynamics　together　with　the　photocatalytic　performance　in　RP.　Two　main　findings　are　achieved:　i)　VP-induced　deep　charge　trapping　effect　is　revealed　to　lead　to　a　severe　loss　of　active　electrons　during　the　H+　reduction　reaction,　resulting　in　the　inferior　PHE　performance　of　RP.　ii)　O　doping　in　the　VP　sites　is　demonstrated　to　be　an　effective　strategy　for　eliminating　the　detrimental　VP　defect　states,　leading　to　a　long-lived　free　electron　lifetime　in　RP　for　enhanced　PHE　performance.　The　point　defect　engineering　of　RP　applied　in　this　study　paves　a　promising　way　in　tuning　the　physicochemical　properties　of　RP　and　other　elemental　-based　materials　for　various　applications.