In this study, a novel process to modify thin-film-composite polyamide membrane was developed. It involved surface treatment of the polyamide membrane by chlorine, followed by supramolecular assembly of chitosan on the membrane surface. While the original polyamide membrane was negatively charged, the surface modification led to a charge reversal due to skin layer of chitosanium in the resulting polyamide/chitosan composite membrane. The polyamide/chitosan (PA/CS) composite membrane was shown to perform better than the original polyamide membrane. The parameters involved in the chlorination of polyamide (e.g., chlorination time and chlorine concentration) and supramolecular assembly of chitosan (e.g., concentration and deposition time) on the membrane surface were investigated. The following conditions for membrane modification were found to be appropriate: NaClO concentration 200 mg/L, chlorination time 2-5 min, and chitosan concentration 1000 mg/L; the PA/CS membrane so formed exhibited a permeation flux of 57.7 L/(m 2 h) and a salt rejection of 95.4% for a feed NaCl concentration of 1500 mg/L at 0.8 MPa. The PA/CS composite membrane also exhibited good performance for rejection of divalent salts (99.8% for MgCl 2 and 98.5% for Na 2 SO 4 ) at the same concentration. This modification technique is simple and practical because dilute solutions are used for surface treatment and commercial membrane units can be modified in their original modules.