A　mathematical　model　is　established　to　describe　a　hollow　fiber　membrane　separator　by　binary　gases　which　including　water　vapor　under　low　feed　gas　pressure　and　vacuum　operation.　The　permeability　of　water　vapor　is　described　by　resistance　model　in　each　differential　unit　and　the　boundary　resistance　caused　by　concentration　polarization　is　taken　into　consideration.　Industrial　gas　acetylene　dehydration　by　PSF　hollow　fiber　composite　membrane　is　used　to　test　the　model.　The　simulated　results　show　good　agreement　with　the　experimental　data.　The　calculated　results　will　bring　50%　error　of　the　water　vapor　composition　of　the　retentate　gas　when　concentration　polarization　is　ignored.　The　permeability　of　water　vapor　is　decreased　slowly　along　the　fiber　from　the　entrance　of　the　feed　gas　to　the　exit,　mainly　because　of　the　water　vapor　partial　pressure　changes　with　the　length　of　the　fiber.　The　influence　of　concentration　polarization　on　separation　is　also　discussed　by　this　model.　The　actual　separation　factor　alpha　H(2)O/C(2)H(2)　less　than　1/5　to　the　intrinsic　separation　factor　of　the　composite　membrane　due　to　the　concentration　polarization.　The　concentration　polarization　of　the　feed　side　boundary　has　little　influence　on　the　membrane　separation　process.　The　permeate　side　boundary　resistance　controls　the　whole　mass　transfer　in　the　separator.