Antifouling performance of membranes is the key obstacle limiting their practical applications for oil/water separation. In this study, a sustaining antifouling membrane was fabricated by constructing polydopamine (PDA) micro-/nano-spheres on a polyacrylonitrile (PAN) nanofibrous membrane. The secondary PDA nano-spheres not only strengthened the bonding of primary micro-spheres with the substrate, but also diversified the hierarchical structure and chemistry. The composite showed enhanced superhydrophilicity and underwater superoleophobicity. Permeability of PAN-PDAc membrane was maintained as high as 11666 +/- 978 Lm(-2)h(-1)bar(-1) with separation efficiency of higher than 99.9% over a 2-h continuous filtration. This permeability was about 2.7 times of pristine PAN membrane (4260 +/- 430 Lm(-2)h(-1)bar(-1)). The extrusion and cutting demulsification on the confined space of PAN-PDA surface was proposed. Antifouling mechanism of the superhydrophilic membrane was first theoretically elucidated based on hydration ability and adhesion free energy with recourse to thermal analysis and Derjaguin-Landau-Verwey-Overbeek theory respectively. It was found that PDA micro-/nano-spheres mediated membrane showed strong hydration ability (higher fraction of non-freezable water) and weak adhesion towards toluene (low free energy of adhesion) compared to pristine PAN membrane. These findings would lead to a better understanding of antifouling demulsification mechanism and improved design of sustaining antifouling membranes for oil/water separation.