Bismuth　oxyhalides　(BiOX),　as　a　typical　photocatalytic　material,　have　attracted　much　attention　due　to　their　unique　layered　structure,　non-toxicity　and　excellent　stability.　However,　the　photocatalytic　performance　of　BiOX　is　limited　by　their　weak　light　absorption　ability　and　rapid　recombination　of　photo-generated　carriers.　In　the　present　work,　first-principles　calculations　have　been　performed　to　comprehensively　explore　the　structural,　electronic　and　optical　properties　of　black　phosphorus　(BP)/BiOX　(X　=　Cl,　Br,　I)　heterostructures,　revealing　the　inherent　reasons　for　their　enhanced　photocatalytic　performance.　By　combining　band　structures　and　work　function　analysis,　the　migration　paths　of　photo-generated　electrons　and　holes　are　obtained,　proving　a　direct　Z-scheme　photocatalytic　mechanism　in　BP/BiOX　heterostructures.　Moreover,　the　BP/BiOX　heterostructures　have　decent　band　edge　positions,　which　are　suitable　for　photocatalytic　overall　water　splitting.　Compared　with　single　BiOX,　the　light　absorption　performance　of　BP/BiOX　heterostructures　is　significantly　improved,　in　which　BP/BiOI　exhibits　the　highest　optical　absorption　coefficient　among　the　BP/BiOX　heterostructures.　Meanwhile,　the　better　carrier　migration　performance　of　the　BP/BiOX　heterostructures　is　attributed　to　the　reduction　in　effective　mass.　The　present　work　offers　theoretical　insight　into　the　application　of　BP/BiOX　heterostructures　as　prominent　photocatalysts　for　water　splitting.