Expected to be the next generation separation membranes, graphene-based functional membranes are springing up in the domains of water desalination and purification. Considering the relatively large interlayer spacing and the swelling effect of the pure GO membranes in solution, feasible strategies must be adopted to overcome these obstacles. Based on the electrostatic attraction between GO and protonated g-C3N4, uniform and compact GO/g-C3N4 hybrid membranes are firstly prepared by a vacuum filtration method and then rGO/g-C3N4 hybrid membranes are fabricated under mild UV irradiation. During the in situ photocatalytic reduction of GO, oxygen functionalities are gradually removed by virtue of g-C3N4 without any chemicals, resulting in the decreased interlayer spacing, the restrained swelling effect of the hybrid membranes and the weakened electrostatic screening of ion hydration shells, and then the outstanding water desalination is obtained. As-prepared rGO/gC(3)N(4) hybrid membranes irradiated under UV for 18 h show high rejections towards several ion salts under maintaining the appropriate water flux, especially for sodium salts (67.5% for NaCl, 89.2% for Na2SO4). Based on characterization results, the reason of the difference of the rejections towards various salts and the desalination mechanisms are discussed. This work offers valuable clues for preparing all-carbon based membranes.