A new sulfonated aromatic diamine monomer, potassium 2,5-bis(4-aminophenoxy)benzenesulfonate (BAPBS), was synthesized and employed to develop a series of thin-film composite (TFC) nanofiltration membranes with trimesoyl chloride (TMC) on a polysulfone (PSF) substrate by an interfacial polymerization (IP) technique. The TFC membrane performed a high water flux of 72.8 L m(-2) h(-1) and a rejection of 92.5% to Na2SO4 at 0.6 MPa. The surface hydrophilicity of the as-prepared sulfonated polyamide (SPA) membrane was greatly improved by the introduction of sulfonic acid groups, as confirmed by the much reduced contact angle value. Moreover, the membrane also exhibited good antifouling ability with water flux recovery ratio (FRR) and total flux decline ratio (DRt) of about 88% and 18%, respectively. Molecular dynamics simulation was investigated to obtain an in-depth understanding of the transport behaviors of water molecules through the SPA polymers. The results clearly illustrated that the diffusion coefficient of water molecules in the sulfonated membrane matrix was about 21% greater than that in the nonsulfonated one. Overall, the combined results suggest that this type of SPA nanofiltration membrane is a promising candidate for water softening and purification applications.