The influences and treatments of oil/water mixtures have been plaguing people for many years. To deal with it, the fabrication of separation membranes becomes a significant technology in the application to oily wastewater treatment. Except for the well-known phase inversion method which is promising for polymeric separation membrane fabrication, another imaginable method referred to as in-situ elimination here still suffers from major difficulty in the uniform dispersion of dissoluble micro/nano-particles in polymeric matrix. Aiming at in-situ elimination method, a series of free-standing thin PVDF membranes with interconnected porous structures are fabricated based on low-cost starting materials through a simple process of tape-casting, drying and immersing, in the presence of pore-forming agent of citric acid monohydrate (CAM). Substantially, the pore formation mechanism is studied, which is resulted from the recrystallization of pore-forming agent (CAM) and subsequent in-situ elimination in NaHCO₃ solution. The fabricated free-standing porous PVDF membrane with thickness of ~5 µm and surface pore size of <500 nm showed great flexibility and superoleophilic, under-oil superhydrophobic, anti-water-fouling properties. For the porosity and selective wettability, their applications in oil/water separation are studied also and the results show that the PVDF membrane can separate not only immiscible oil/water mixtures quickly but also various surfactant-stabilized water-in-oil nanoscale emulsions, with filtration flux up to 318 L m⁻² h⁻¹ and separation efficiency of 99.81% under the action of self-gravity. To the best of our knowledge, this is the first time the polymeric separation membranes have been positively fabricated by in-situ elimination, which provides a new avenue for porous polymeric membrane fabrication in membrane science field and shows the application in oil/water or emulsion separation. This work provides a new porous membrane-forming method and an energy-efficient strategy for separating immiscible oil/water mixtures and surfactant-stabilized water-in-oil nano-emulsions.