The　complex　internal　flow　field　of　steam　ejector　was　numerically　simulated　with　commercial　software　FLUENT.　The　pressure　and　velocity　variation　of　internal　fluid　in　the　flowing　process　were　studied　and　analyzed.　The　influences　of　the　mixing　steam　pressure　and　the　outlet　diameter　of　primary　nozzle　on　the　ejector　performance　of　the　steam　ejector　were　discussed　in　a　great　length.　It　is　found　from　the　results　that　the　entrainment　ratio　of　the　steam　ejector　remains　constant　first　with　the　back　pressure　to　its　critical　pressure　and　then　decreases,　nevertheless,　the　entrainment　ratio　initially　increases　sharply　with　the　primary　nozzle　outlet　diameter　and　then　decreases　after　the　outlet　diameter　exceeds　a　certain　value.　Therefore　there　exists　a　critical　outlet　pressure　for　the　steam　ejector　that　determines　its　operation　states.　The　outlet　diameter　of　main　nozzle　has　an　optimal　range,　in　which　the　ejector　obtains　the　best　performance,　and　the　dissipation　loss　of　the　shock　wave　is　relatively　small.　©　All　Right　Reserved.