A water vapor induced phase separation (WVIPS) electrospinning process for the preparation of core-shell nanofibers with a single capillary is described. The PAN-PVP core-shell nanofibers were fabricated by WVIPS electrospinning a polyvinylpyrrolidone (PVP)-polyacrylonitrile (PAN) homogeneous solution. The solvent solubility parameter of the liquid electrospinning jets was altered by water vapor, which subsequently induced the phase separation before the solvent rapidly volatilized, and caused the formation of core-shell structured nanofibers. A scanning electron microscope (SEM) and transmission electron microscope (TEM) were applied to characterize the morphology. X-ray photoelectron spectroscopic analysis (XPS), water contact angle (CA) tests and thermogravimetry analysis (TGA) were used to confirm the composition of the fiber's shell. The results of TGA also showed an improvement in the thermal stability of the fiber membranes.