Membrane fouling caused by surfactant-stabilized oil-in-water emulsions is a great challenge for oily water treatment. In this work, we fabricated a sulfobetaine-based zwitterionic nanogels modified polyacrylonitrile (ZPAN) nanofibmus membrane with superwetting surfaces. The zwitterionic nanogels were grafted on the surface of electrospun PAN nanofibers to construct hierarchical structure which can enhance surface roughness and antifouling performance of the membrane. The ZPAN membrane possesses superhydrophilic and underwater superoleophobic properties under a wide range of salt concentrations and pH values. Driven solely by gravity (applied pressure = 1 kPa), the ZPAN membrane can successfully separate oil and water emulsions. The oil content in the collected filtrates was below 20 ppm with a high separation efficiency of >99.6%, meanwhile, the permeation fluxes can reach 32800 L m(-2) h(-1) bar(-1). Moreover, the ZPAN membrane exhibit strong mechanical property and stable reusability, indicating a great potential for efficient low-pressure-driven separation of oil-in-water emulsions.