Membrane fouling and degradation caused by free chlorine oxidation seriously hinder the long-term operation of aromatic polyamide (PA) thin-film-composite (TFC) membranes. In this study, sulfonamide monomers 4-aminobenzene sulfonamide (4-ABSA) and 2-aminoethanesulfonamide (2-AESA)) were facilely grafted onto the PA RO membranes surface via layer-by-layer interfacial polymerization (LbL-IP) method, endowing with much improved water flux, antifouling, and chlorine-resistance properties. The chemical compositions and surface properties of the RO membranes were systematically investigated by various techniques, as FTIR and XPS results clearly showed that sulfonamide groups were successfully grafted onto RO membranes’ surface. Compared to the pristine membrane, the water flux of the two modified membranes increased by 50.8% and 59.1%, reached 3.65 ± 0.15 L m⁻² h⁻¹ bar⁻¹ and 3.85 ± 0.05 L m⁻² h⁻¹ bar⁻¹, respectively, meanwhile their NaCl rejection remained above 99.25 ± 0.08%. Due to the improvement of surface hydrophilicity and decrease of surface roughness, the antifouling performance of the modified membranes was greatly improved. Moreover, the N–H moieties on sulfonamide can act as sacrificial units for free chloride attack to significantly improve the chlorine-resistance performances of the modified RO membranes compared to the pristine one, with NaCl rejection remain above 98.95 ± 0.09% after chlorination intensity of 8000 ppm⋅h under acidic condition of pH 4. This simple and efficient surface grafting strategy and modifying materials make it have great application potential in the field of water treatment.