The　existence　of　back　pressure　at　outlet　has　a　direct　impact　on　the　separation　performance　of　the　separation　separator.　However,　the　influence　of　back　pressure　is　not　understood　well　due　to　the　complicated　interaction　of　the　supersonic　flow,　condensation　and　shock　waves.　In　this　study,　the　condensation　flow　models　of　water　vapor　were　established,　the　influence　of　back　pressure　on　supersonic　condensation　properties　of　water　vapor　was　investigated.　The　results　show　that　when　the　back　pressure　exists,　the　position　of　the　shock　wave　is　more　forward　than　that　without　condensation,　and　the　strength　of　shock　wave　is　weaker.　The　nucleation　rate　is　generated　around　the　position　of　x　=　115　mm　and　increases　sharply　from　0　to　10(25)　m(-3　)s(-1).　The　shock　wave　is　generated　at　x　=　145　mm　and　x　=　122　mm　when　the　outlet　back　pressure　is　20　kPa　and　30　kPa,　respectively.　The　condensed　droplets　evaporate　again　with　the　effect　of　shock　wave,　and　then　the　secondary　condensation　of　droplets　occurs　owing　to　the　cooling　effect　of　nozzle.　When　the　back　pressure　is　higher　than　a　certain　value,　the　refrigeration　environment　in　the　nozzle　is　completely　destroyed　with　the　effect　of　shock　wave,　and　the　condensed　droplets　evaporate　again.