Supercritical　compressed　air　energy　storage　system　requires　high　turbine　efficiency　over　a　wide　working　range　at　both　the　design-point　and　off-design　point.　The　operating　range　of　the　turbine　is　often　limited　by　the　occurrence　of　flow　instability,　such　as　distinct　vortex　and　load　deterioration.　In　specific　situations,　the　development　of　aerodynamic　instabilities　at　low　inlet　pressure　operating　conditions　can　lead　to　lower　turbine　efficiency,　even　lead　to　turbine　choke,　limiting　the　wide　operating　range　of　the　turbine.　Specifically,　the　high-pressure　stage　turbine　of　the　expander,　namely　the　first　stage　turbine,　will　not　only　bears　a　sharp　change　of　inlet　pressure　but　also　experiences　the　maximum　and　variable　pressure　drop.　In　this　paper,　Real　Gas　Property　(RGP)　file　for　supercritical　air　is　used　for　simulations.　The　CFD　method　and　RGP　file　are　validated　by　the　turbine　in　the　NASA　report.　A　detailed　3D　CFD　analysis　is　performed　for　the　preliminary　designed　first　stage　turbine　under　variable　inlet　pressure.　With　the　purpose　of　improving　the　aerodynamic　performance　of　the　first　stage　turbine　under　extreme　working　conditions,　a　series　of　simulations　are　conducted　to　examine　the　effects　of　the　IGV　parameters　(i.e.　different　types　of　IGVs,　the　nozzle-impeller　radial　gap　distance　and　the　nozzle　blade　installation　angle)　and　blade　thickness,　which　led　to　the　optimization　in　overall　turbine　stability.　The　results　show　that　IGV　TC-2P　have　good　aerodynamic　performance,　and　the　matched　rotor　can　achieve　an　isentropic　efficiency　of　80%　under　off-design　working　conditions.　The　optimal　blade　installation　angle　blade　is　35°.　And　there　exists　an　optimal　nozzle-impeller　radial　gap　distance　Δr.　The　turbine　efficiency　obtains　a　maximum　value　at　Δr　=　5　mm.　In　all　cases　for　blade　thickness,　the　different　configurations　have　a　significant　change　in　inlet　total　pressure　of　4.5　MPa　compared　to　the　design　pressure　of　7　MPa.　The　maximum　efficiency　is　reached　at　the　ellipse　ratio　is　1:1　for　Case-2.　The　results　of　the　optimal　IGV　TC-2P　performance　optimization　is　essential　to　improve　the　isentropic　efficiency　at　low　inlet　pressure.　Copyright　©　2019　ASME.