Poly　(triazine　imide)　(PTI),　a　crystalline　allotrope　of　polymeric　carbon　nitride,　holds　great　promise　for　photocatalytic　overall　water　splitting　(OWS).　However,　previous　results　reveal　that　both　H-2　and　O-2　evolution　cocatalysts　prefer　to　deposit　on　the　prismatic　facets,　which　results　in　the　undesired　backward　reaction　and　current　unsatisfied　quantum　efficiency　for　OWS　reaction.　Herein,　in　situ　decoration　with　heptazine　based　carbon　nitride　(HCN)　at　the　interface　of　PTI　is　successfully　employed　to　construct　S-scheme　hetero-phase　junctions.　The　built-in　electric　field　at　the　interface　of　PTI　and　HCN　endows　directed　carrier　separation,　thereby　enabling　the　separation　of　redox　sites,　as　validated　by　theoretical　and　experimental　results.　Accordingly,　the　as-synthesized　PTI/HCN　with　spatially　separated　charge　carriers　and　restrained　backward　reaction　achieves　a　solar-to-hydrogen　energy　conversion　efficiency　of　0.35%　for　photocatalytic　OWS　reaction.