Conventional　polymerization　for　the　synthesis　of　carbon　nitride　usually　generates　amorphous　heptazine-based　melon　with　an　abundance　of　undesired　structural　defects,　which　function　as　charge　carrier　recombination　centers　to　decrease　the　photocatalytic　efficiency.　Herein,　a　fully　condensed　poly　(triazine　imide)　crystal　with　extended　π-conjugation　and　deficient　structure　defects　was　obtained　by　conducting　the　polycondensation　in　a　mild　molten　salt　of　LiCl/NaCl.　The　melting　point　of　the　binary　LiCl/NaCl　system　is　around　550　°C,　which　substantially　restrain　the　depolymerization　of　triazine　units　and　extend　the　π-conjugation.　The　optimized　polymeric　carbon　nitride　crystal　exhibits　a　high　apparent　quantum　efficiency　of　12　%　(λ=365　nm)　for　hydrogen　production　by　one-step　excitation　overall　water　splitting,　owing　to　the　efficient　exciton　dissociation　and　the　subsequent　fast　transfer　of　charge　carriers.　©　2021　Wiley-VCH　GmbH