Poly(vinylidene fluoride) (PVDF), polymethyl methacrylate (PMMA) and cellulose acetate (CA) in the presence of N-methyl pyrrolidone were blended in various combinations by using dimethylformamide (DMF) and glacial acetic acid (CH3COOH) as solvent and used to prepare flat-sheet membranes by the phase-inversion process. A square-crossing table with four factors and three levels for the PVDF-PMMA-CA system was designed to study the influences of the casting solution composition and gelation conditions on membrane pore size; preparation conditions for the system were optimized. The results showed that the effect of the casting solution composition was stronger than that of the solid content and the content of the coagulation medium on characteristics of the prepared membrane. The suitable composition for the PVDF-PMMA-CA system was found to be a solid content of 18 %, an additive content of 4% , DMF/CH3COOH = 4:1 and a NaCO, solution with aa concentration of 2 % as the coagulation medium. In addition, it was found that carbon dioxide gas produced by the reaction of glacial acetic acid in the cast solution with sodium carbonate in a coagulation bath can be used to control the even distribution of the membrane pore size during the process of coagulation.