Graphitic carbon nitride (g-C3N4) has been applied extensively in many fields, where there often shows synergistic intermolecular interaction to promote reaction process. In this paper, the dimolecular interaction between g-C3N4 and phenols (picric acid (PA), pentachlorophenol (PCP), phenol (PhOH)) was examined by photoluminescence quenching of g-C3N4. The spectroscopy analysis, thermodynamic analysis combined with theoretical simulation helped to reveal mechanism. UV-vis absorption spectroscopy indicated that the interaction mechanism was charge-transfer. The fluorescence quenching constants calculated from Stern–Volmer equation showed weak binding complexes between g-C3N4 and phenols were formed. The interaction affinity decreased in the order of PA > PCP > PhOH. The enthalpy change (ΔH) and entropy change (ΔS) calculated from Van't Hoff equation showed that electrostatic interaction was the essential force in stabilizing g-C3N4–PA, while hydrogen bond was the essential force in stabilizing g-C3N4–PCP and g-C3N4–PhOH. Theoretical simulation showed that the –N– residue or the cavity of g-C3N4 was the preferential active site. It is the auto-doped N atoms in g-C3N4 nanosheets that lead to region charge polarization and introduce active sites for adsorption of molecules.