This communication reported a new simple method to produce porous ultrafine fibers during electrospinning process. It was named liquid-liquid phase separation method. This method required introducing suitable amount of water into electrospun solution to form similar stable three-phase membrane-forming systems previously. Scanning electron microscopy was employed to evaluate the morphology of porous ultrafine fibers. According to the investigation result, porous ultrafine fibers were prepared by this method. Liquid-liquid phase separation caused by the change of the property of three-phase systems of charged jet that was led by the rapid solvent evaporation during electrospinning process was considered as the key role for the formation of porous structure. This method is very simple and it not only lowers the requirement on the solvents but also does not introduce any new component into the fibers. Therefore, this method provides a new simple way to form porous structure during electrospinning process.