Constructing　S-scheme　heterojunctions　is　widely　recognized　as　an　effective　strategy　to　enhance　the　photocatalytic　overall　water-splitting　performance　of　polymeric　carbon　nitride.　However,　carbon　nitride-based　S-scheme　heterojunctions　are　frequently　constrained　by　a　low　lattice　matching　degree,　weak　interfacial　adhesion,　and　high　interfacial　trap　density,　which　significantly　impede　interface　electron　transport　efficiency.　To　address　these　limitations,　we　fabricate　a　crystalline　Rb　doped　polyheptazine　imide　(Rb-PHI)/melon　(P&M)　homojunction　with　tiny　lattice　mismatches　by　the　flux-assisted　synthesis　method.　Structural　characterizations　confirm　that　the　flux-assisted　synthesis　method　can　effectively　regulate　the　interface　structure　between　PHI　and　melon,　thus　forming　a　crystalline　and　coherent　homointerface.　Moreover,　kinetic　analysis　reveals　that　fabricating　a　nanoscale　lattice-matched　homointerface　can　generate　a　robust　interfacial　electric　field,　thereby　facilitating　the　directed　migration　and　spatial　separation　of　photoexcited　electrons　and　holes.　More　importantly,　we　find　that　the　photocatalytic　overall　water-splitting　activity　and　interfacial　charge　transfer　efficiency　of　P&M　homojunction　depend　on　the　degree　of　interfacial　lattice　matching　and　crystallinity.　This　study　underscores　the　significance　of　interfacial　structure　for　enhancing　the　charge　transfer　efficiency　in　S-scheme　homojunctions　and　offers　valuable　insights　and　methodologies　for　developing　high-performance　S-scheme　homojunctions.　©　2025　American　Chemical　Society