The　Frenkel　exciton　dissociation　efficiency　and　the　subsequent　free　charge　carrier　migration　rate　mainly　determine　the　photocatalytic　property　of　polymeric　carbon　nitride　(PCN).　Herein,　by　postcalcination　of　heptazine-based　PCN　in　molten　salts　at　mild　temperature　(673　K),　we　are　able　to　design　a　series　of　crystalline　PCNs　with　a　promising　triazine-heptazine　hybrid　structure.　The　molecular　triazine-heptazine　junctions　promote　the　exciton　splitting,　while　the　ordered　in-plane　packing　structure　facilitates　the　hot　charge　carrier　migration.　Accordingly,　the　optimal　triazine-heptazine-based　PCN　exhibits　a　dramatically　enhanced　activity　toward　one-step　excitation　photocatalytic　overall　water　splitting　to　generate　H2　and　O2　with　visible　light　over　heptazine-based　pristine　PCN,　by　a　factor　of　∼34　times.　©　2021　American　Chemical　Society.　All　rights　reserved.