The　specific　process　of　membrane　formation　via　water-vapor　induced　phase　separation　method　was　investigated　through　observing　the　gelation　process　of　polyvinylidene　fluoride(PVDF)　casting-solution　in　vapor　under　optical　microscope,　combining　mass　variation　of　the　membrane　solution　under　different　humidity　and　infrared　spectra　of　the　liquid　drop　layer　on　the　film　surface.　The　results　showed　that　there　existed　transfer　between　solvent　and　non-solvent　during　the　membrane　formation.　The　film　quality　increased　in　the　beginning　due　to　the　absorption　of　non-solvent　water　as　dominant.　Afterwards,　film　quality　reached　a　maximum　value,　then　decreased　gradually　to　be　steady　owing　to　the　volatilization　of　the　solvent　and　non-solvent　as　dominant.　The　gelation　rate　in　the　high　humidity　was　faster　than　that　in　the　low　humidity　because　of　the　faster　exchange　between　the　solvent　and　non-solvent.　According　to　the　SEM　images　of　the　PVDF　membrane　surface　and　cross-section,　the　final　membrane　structure　for　rough　and　porous　surface　was　formed　with　the　longer　exposure　time　in　vapor　while　the　symmetrical　cross-section　formed.