Surface-initiated atom transfer radical polymerization (SI-ATRP) can precisely control the graft length and graft density of polymers. In this work, nonfouling nanofiltration membrane with tunable zwitterionic layer was fabricated by grafting sulfobetaine methacrylate (SBMA) on the surface of 2-bromoisobutyryl bromide (BIBB)-immobilized polyamide membranes (PA-Brs) via SI-ATRP. Zwitterionic layer with different graft length and graft density can be adjusted by varying the BIBB concentration or ATRP time, which can thus tune water flux and antifouling property of the membranes. When BIBB concentration is 1.00 wt% and ATRP time is 0.2 h, the optimal flux is 63.9 Lm(-2) h(-1). In addition, the amount of foulant adsorbed on membrane is less than 20 mu g cm(-2). Flux decline ratio is about 5% and flux recovery ratio is more than 98% when humic acid is used for dynamic test, suggesting a nonfouling performance of zwitterionic membrane with SBMA graft content above 26 mol%.