The　geometry　and　operation　parameters　have　important　influences　on　the　performance　of　steam　ejectors.　An　axisymmetric　two-dimensional　mathematical　model　for　transonic　compressible　flow　inside　a　steam　ejector　has　been　established　in　this　paper　to　investigate　the　flow　characteristics　inside　steam　ejectors　aiming　at　optimizing　primary　steam　nozzle　exit　section　distance　and　mixing　chamber　throat　diameter　simultaneously.　The　results　show　that　there　are　an　optimum　value　for　the　primary　steam　nozzle　distance　ratio　and　an　optimum　diameter　ratio　of　mixing　chamber　throat　to　primary　nozzle　throat　at　which　the　steam　ejector　acquires　its　best　entrainment　performance　under　the　given　design　conditions.　With　the　optimized　primary　steam　nozzle　distance,　the　optimization　of　the　diameter　ratio　of　the　mixing　chamber　throat　to　primary　nozzle　throat　is　significant.　In　our　case,　the　improvement　of　the　entrainment　ratio　is　as　large　as　25%.　Deviation　from　its　optimized　value　may　result　in　a　serious　degeneration　in　the　ejector　performance.　Therefore,　to　ensure　the　steam　ejector　performance,　the　primary　steam　nozzle　distance　should　be　designed　at　its　optimum　value　and　the　diameter　ratio　of　the　mixing　chamber　throat　to　primary　nozzle　throat　be　within　a　narrow　vicinity　of　its　optimum　value.