Solar-driven　photocatalytic　overall　water　splitting　represents　a　sustainable　strategy　to　produce　green　hydrogen　using　metal-free　polymeric　carbon　nitride.　However,　conventional　thermal　polymerization　with　a　single　precursor　for　the　synthesis　of　poly　(triazine　imide)　faces　challenges　such　as　slow　deamination　rates　and　mass　transfer,　resulting　in　the　formation　of　undesired　structural　defects,　which　usually　serve　as　charge　recombination　sites　and　decrease　the　photocatalytic　performance.　Herein,　highly　crystalline　poly　(triazine　imide)　by　thermal　copolymerization　of　binary　precursors　of　melamine　and　cyanuric　acid　in　the　presence　of　molten　salts　is　reported.　The　results　reveal　that　melamine　and　cyanuric　acid　easily　generate　melam　(intermediates　of　carbon　nitride)　and　subsequently　facilitate　the　polycondensation　process.　Solid　characterizations　revealed　that　crystalline　poly　(triazine　imide)　nanoplates　with　extended　π-conjugation　degrees　and　reduced　intensity　of　structural　defects　would　be　obtained,　which　largely　promotes　separation　and　migration　of　photogenerated　carriers,　and　inhibits　the　undesirable　charge　recombination　at　the　defect　sites.　Accordingly,　after　in　situ　photo-deposition　of　CoOx　and　Pt/Cr2O3　as　O2　and　H2　evolution　co-catalysts,　respectively,　the　optimized　crystalline　poly　(triazine　imide)　nanoplates　achieve　an　excellent　solar-to-hydrogen　conversion　of　0.30%　under　simulated　sunlight　irradiation.　©　2024　Wiley-VCH　GmbH.