Polyaniline (PANI) decorated commercial filtration membranes, such as stainless steel meshes (SSMs) with 5 μm pore size and polyvinylidene fluoride (PVDF) membranes with 2–0.22 μm pore sizes, were fabricated by a simple one-step dilute polymerization at low temperature. Lots of short PANI nanofibers were firmly and uniformly coated onto the membrane surfaces, forming rough micro- and nanoscale structures and leading to underwater superoleophobicity with low oil-adhesion characteristic. Furthermore, we systematically studied the effect of pore size and pressure difference on oil–water separation ability of the obtained membranes. It was found that the PANI-modified SSMs with 5 μm pore size were suitable for the separation of non-surfactant emulsions with water fluxes of more than 1000 L m⁻² h⁻¹ under gravity only. The PANI-modified PVDF membranes were used for the effective separation of surfactant-stabilized emulsions with water fluxes up to 3000 L m⁻² h⁻¹ for 2 μm pore size under 0.1 bar or 0.22 μm pore size under 0.6 bar. In addition, the superhydrophilic membranes with PANI coatings were demonstrated for high oil rejection, stable underwater superoleophobic properties after ultrasonic treatment and immersing in oils and various harsh conditions, and high and steady water permeation flux after several cycles.