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Nanofiltration (NF) membranes exhibit great performance deterioration when contacting with high concentration chlorine solutions, which is unbeneficial to their sustainable operation. Herein, a chlorine-stable melamine was surface-grafted to the polyamide membrane by a covalent modification method to improve the anti-chlorine property of NF membranes. The grafting of melamine significantly changed the surface features of NF membranes. The grafted membranes owned rougher morphology, narrower pore size, higher hydrophilicity, and greater positive charge compared to the unmodified membrane. Higher salt rejections (NaCl, MgCl₂, MgSO₄, and Na₂SO₄) with slightly declined water permeability were obtained for the grafted membranes. Importantly, the optimized membrane with melamine grafting could maintain a high separation performance when contacting with 2000 ppm NaClO solution for 10 h at a …