Theoretical ternary phase diagrams in good agreement with experiment cloud point data of water/N,N-dimethylacetamide (DMAc)/polyamic acid (PAA), ethylene glycol (EG)/DMAc/PAA and ethanol/DMAc/PAA systems were constructed based on the extended Flory-Huggins theory of the polymer solutions. Binary interaction parameters were calculated and optimized accurately to construct the theoretical phase diagrams. The surface and cross-section microstructures of the as-spun fibers were analyzed by the dual diffusion theory and phase separation mechanism based on the ternary diagrams. With the concentration of DMAc in coagulation increasing, sponge-like and less skin-core structures with rougher surface were observed for fibers prepared from water/DMAc/PAA system, as the homogeneous region and metastable two-phase region enlarged in this case and it favors the nucleation and growth demixing mechanism. Further increasing concentration of DMAc in coagulation resulted in the formation of porous structures. On the other aspect, fibers prepared from EG/DMAc/PAA system exhibited a noncircular cross-section and an obvious skin-core structure, while circular cross-section and dense structures were obtained in fibers from ethanol/DMAc/PAA system. The results are in accordance with predicted ones based on theoretical ternary phase diagram.