Carbon nitride shows great potential in photocatalysis, to some extent, due to its insolubility and high thermal, chemical and optical stability. However, it always brings great difficulties for its surface modification and hinders the photocatalysis development. Traditional salt without melting condition is hard for the direct modification of carbon nitride. Herein, pristine carbon nitride (CN) was further modified through the synergetic modification of halide salts (NaCI and KCl) and water vapor. For the obtained product (Na-K-4h), it was carefully studied from three points: the appearance and morphology, the change of chemical structure and composition, and the chemical analysis of its by-products. These researches reveal that H2O could be divided into H and OH. H helps the halide away from halide salt, while the left OH replaces NH groups and help K and Na chelate with CN. This method could also be applied to other salt mixture system (such as NaCl/KI, NaCl/KBr). In this work, Na-K-4h was further applied in photocatalystic H-2 evolution (PHE). Based on this synergistic effect, Na-K-4h show about 27-fold PHE activity higher than CN.