The role played by acetic acid in the formation of poly(vinyl alcohol) (PVA) membranes was investigated. Membranes were prepared from a casting solution of PVA, water, and acetic acid by immersion in Na2SO4/KOH/H2O coagulation bath. Experimental results show that the acetic acid additive exerts an influence on the structure and filtration properties of membranes. Not only the surface morphology but also the structure of cross-section could be modulated by adding the acetic acid in the casting solution. Obviously, the increase of the amount of acetic acid in the casting solution decreased the thickness of skin layer. This could be attributed to the fact that the increase of the amount of acetic acid in the casting solution increases H3O+ ion of the casting solution enhancing the influx rate of coagulant medium for acid-base equilibrium. A mechanism describing the affinity between the PVA solution and the coagulant medium is proposed to estimate the PVA membrane structure by adding the acid. The results presented here offer a better understanding of relationships between the membrane formation mechanism and the skin structure when designing an asymmetric membrane with acetic acid as an additive.