The performance of nanofiltraton (NF) membranes are characterized by a high selectivity for multivalent anions, whereas poor selective separation capacity for multivalent cations and heavy metal ions. In this work, a new positively charged NF membrane was prepared by using 2-chloro-1-methyliodopyridine as an active agent to graft polyimide polymer onto the membrane surface via covalent bonding with surface carboxylic groups. The results showed that the prepared membranes had a high selective separation for divalent and monovalent salts (MgCl2 97.1%, CaCl2 96.8%, Na2SO4 83.5%, MgSO4 87.3%, NaCl 49%, LiCl 32%), and also exhibited a high removal efficiency for toxic heavy metal ions and dyes (CuCl2 96%, NiCl2 95.8%, CrCl3 98.0%, Tropaeolin O 98.3%, Victoria blue B 99.2%, Semixylenol orange 99%, Neutral 98.2%).