This paper presents an original approach to study the dependence of casting conditions in wet phase inversion process and their influence on the membrane morphology and separation performance of water/ethanol mixture by pervaporation. The casting conditions of polysulfone membrane were carried out with various coagulation temperatures and polymer concentration in water bath. It was found that the asymmetric polysulfone membranes showed a strongly influence on the skin layer structure and the separation performance in pervaporation. The decrease in skin layer thickness of asymmetric was obtained by increasing the coagulation temperature due to the decline in demixing rate of non-solvent in the coagulation bath. On the other hand, the polymer concentration also dominated the membrane formation path ways in wet phase inverse method. In this study, the lowering in polymer concentration in polysulfone/NMP/water system, the defective skin layer was obtained and presents a poor separation performance. Based on the morphology observation, light transmittance measurement, and pervaporation test, the asymmetric polysulfone membrane with defect free skin layer can be obtained by the predominated casting conditions and polymer concentration. This study is successful to prepare a defect free skin layer asymmetric polysulfone membranes by controlling the coagulation temperature and polymer concentration in casting solution. The SEM observations found that the defect free skin layer was formed by lowering the temperature in coagulation in wet phase inverse process. On the other hand, an increasing in polymer concentration obtained the same defect free skin layer structure of asymmetric membranes in wet phase inverse process. The pervaporation performance of those membranes confirmed the morphology observation and presented a good performance. The light transition measurements were used to verify the change of membrane formation path ways of those membranes. For preparing the defect free skin layer asymmetric membrane, this study indicated that the controlling membranes formation path ways could be easy achieved by lowing the coagulation temperature or increasing the polymer concentration in casting solution. The permeation measurement showed that the permselectivity of asymmetric membrane was dominated by the skin layer morphology. It is suggested that the high performance pervaporation membrane can be prepared by phase inverse method by lowing the coagulation temperature and increasing the polymer concentration in casting solution.