As　core　parts　of　quasi　two-stage　compression　heat　pump　system　coupled　with　ejector,　the　ejector　was　studied　theoretically　with　an　accurate　of　the　ejector　configuration　parameters　to　instruct　the　designing　of　the　ejector　and　to　prove　the　system＇s　working　performance.　The　ejector　model　was　developed　based　on　aerodynamics　function　and　classical　thermodynamics.　The　variations　of　the　performance　of　the　heat　pump　system　with　the　equivalent　diameter　of　nozzle　throat,　proportion　of　acreages　between　imlets　and　outlets　in　convergent　section　of　mixing　chamber　and　acreage　of　mixing　chamber　was　discussed　in　detail.　The　characteristics　of　ejector　under　the　various　operation　conditions　and　the　optimized　design　criteria　of　the　ejector　were　presented.