In　order　to　investigate　the　internal　leakage　characteristics　of　the　expander,　a　mathematical　model　of　the　thermodynamic　working　process　and　a　two-phase　leakage　model　for　gas-oil　flow　were　established.　The　influence　of　clearance　height　on　the　leakage　path　of　different　leakage　path　was　analyzed,　and　the　influence　of　rotational　speed　and　inlet　pressure　on　the　volumetric　efficiency　and　leakage　flow　rate　under　three　organic　fluids　(i.e.　R123,　R134　and　R245fa)　was　compared.　The　results　show　that　the　leakage　through　the　clearance　gap　between　screw　rotor　and　shell　accounts　for　the　most　part,　followed　by　the　leakage　through　the　clearance　gap　between　screw　groove　and　gate　rotor,　and　the　leakage　through　the　clearance　gap　between　gate　rotor　and　shell　is　relatively　small.　The　volumetric　efficiency　increases　with　the　rotational　speed　and　the　inlet　pressure.　The　volumetric　efficiency　of　the　single　screw　expander　using　the　R123　as　the　working　fluid　is　the　best　under　the　same　working　conditions,　followed　by　R245fa　and　R134a.　©　2018,　Editorial　Department　of　Journal　of　Beijing　University　of　Technology.　All　right　reserved.