The　interaction　between　a　gas　molecule　and　photocatalyst　is　vital　to　trigger　photocatalytic　reaction.　The　surface　state　of　photocatalyst　affects　much　in　this　interaction.　Herein,　adsorption　of　H2O　molecules　on　s-triazine-based　g-C3N4　was　thoroughly　studied　by　first-principle　calculation.　Although　various　initial　adsorption　models　with　multifarious　locations　of　H2O　molecules　were　built,　the　optimized　models　with　strong　adsorption　energy　pointed　to　the　same　adsorption　configuration,　in　which　the　H2O　molecule　hold　an　upright　orientation　above　the　corrugated　g-C3N4　monolayer.　An　intermolecular　O–H…N　hydrogen　bond　formed　via　the　binding　of　a　polar　O–H　bond　in　H2O　molecule　and　a　two-coordinated　electron-rich　nitrogen　atom　in　g-C3N4.　Under　the　bridging　effect　of　this　intermolecular　hydrogen　bond,　electrons　would　transfer　from　g-C3N4　to　the　H2O　molecule,　thereby　lowering　the　Fermi　level　and　enlarging　work　function　of　g-C3N4.　Interestingly,　regardless　of　the　substitute,　i.e.　g-C3N4　multilayer,　large　supercell　and　nanotube,　this　adsorption　system　was　highly　reproducible,　as　its　geometry　structure　and　electronic　property　remained　unchanged.　In　addition,　the　effect　of　nonmetal　element　doping　on　adsorption　energy　was　explored.　This　work　not　only　disclosed　a　highly　preferential　H2O　adsorbed　g-C3N4　architecture　established　by　intermolecular　hydrogen　bond,　but　also　contributed　to　the　deep　understanding　and　optimized　design　in　water-splitting　process　on　g-C3N4-based　photocatalysts.　©　2021　Dalian　Institute　of　Chemical　Physics,　the　Chinese　Academy　of　Sciences