The　seasonal　variations　in　condensing　ambient　conditions　have　a　significant　impact　on　the　performance　of　the　organic　Rankine　cycle　system　and　the　operating　conditions　of　the　expander.　Using　a　new　single　screw　expander　with　quasi　two-stage　expansion　is　an　excellent　way　to　ensure　efficient　cross-season　operation　of　organic　Rankine　cycle　under　variable　condensing　ambient　conditions.　In　this　paper,　a　coupled　model　of　quasi　two-stage　expansion　single-screw　expander　and　organic　Rankine　cycle　system　under　variable　condensing　ambient　conditions　is　developed　to　reveal　the　relationships　of　condensing　environment　conditions,　single-screw　expander　performance　and　optimal　operating　point　of　organic　Rankine　cycle.　At　the　same　time,　the　inner　volume　ratio　of　quasi　two-stage　single　screw　expander　is　optimized.　The　results　show　that　the　introduction　of　velocity　expansion　process　can　reduce　the　optimal　inner　volume　ratio　of　the　expander　and　improve　the　optimal　isentropic　efficiency　of　the　expander.　The　optimal　inner　volume　ratio　decreases　from　6.0　to　4.86,　and　the　corresponding　optimal　isentropic　efficiency　increases　from　0.679　to　0.692.　The　net　power　and　the　net　efficiency　of　ORC　both　have　optimal　values.　The　net　power　is　up　to　13.87kW　when　the　wet　bulb　temperature　is　5℃　with　the　inner　volume　ratio　of　4.0,　and　the　net　efficiency　is　up　to　12.66%　when　the　inner　volume　ratio　is　8.0.　The　maximum　net　power　and　net　efficiency　of　ORC　can　be　improved　by　4.51%　and　0.59%　respectively　by　using　the　new　single　screw　expander　with　quasi　two-stage　expansion.　©2023　Chin.Soc.for　Elec.Eng.