A ternary diffusion model has been developed for the evaporation step of the phase inversion process. The model is applied to the analysis of mass transfer dynamics of the evaporation step for the methanol-acetone-cellulose acetate (CA) ternary casting system. The combined analysis of quantitatively computational results from the ternary evaporation model and qualitative dynamic results during the quench process has shown that the evaporation step is essentially necessary to prepare the defect-free, ultrathin skinned asymmetric CA membrane for the separation of CO2/CH4. The skin layer of high CA concentration obtained by evaporation has an ability to suppress liquid-liquid phase separation. And the skin layer with high tensile strength can resist the interfacial tension caused by spinodal decomposition from the substructure. Although the CA concentration in the skin layer increases considerably because of the evaporation step and the following delay time during the quench process, the substructure can still induce the spinodal decomposition because the strong coagulant, methanol, can diffuse rapidly across the ultrathin skin layer. Hence the defect-free, ultrathin-skinned as asymmetric membrane for gas separation con be prepared from methanolacetone-CA casting system by evaporation step and the wet phase inversion.