Photocatalytic　water　splitting　represents　a　promising　approach　for　solar　energy　conversion　and　hydrogen　production.　Polymeric　carbon　nitride　(PCN)　exhibits　potential　but　suffers　from　charge　recombination　and　low　specific　surface　area.　This　study　introduces　a　novel　LaOBr@PCN　one-photon　excited　heterojunction　designed　to　mitigate　these　challenges.　By　anchoring　Melon,　a　PCN　precursor,　onto　LaOBr　through　acid-base　coordination　followed　by　in-situ　thermal　polymerization,　three-dimensional　hierarchical　PCN　nanosheets　are　fabricated　on　LaOBr　microplates.　This　structure　increases　the　specific　surface　area,　exposes　more　active　sites,　enhances　lightharvesting　ability,　and　improves　charge　carrier　separation.　Consequently,　the　optimized　LaOBr@PCN　achieves　hydrogen　and　oxygen　production　rates　of　69.9　mu　mol/h　and　34.7　mu　mol/h,　respectively.　This　work　provides　a　viable　strategy　for　developing　advanced　PCN-based　composite　photocatalysts.