Single-screw　expander　(SSE)　has　the　potential　to　meet　the　high　pressure　ratio　condition　in　small-scale　middle　temperature　ORC　system.　In　this　paper,　variable　internal　volume　ratio　(from　3.00　to　8.00),　new　methods　for　calculating　mass　flow　rate　and　friction　power　are　integrated　into　the　structure-based　SSE　thermodynamic　model.　And　the　maximum　calculation　error　of　mass　flow　rate,　volume　efficiency　and　shaft　efficiency　is　2.8%,　2.1%　and　2.3%　respectively　compared　with　the　experimental　data.　Herein,　the　influence　of　internal　volume　ratio　on　shaft　efficiency,　shaft　power,　volume　efficiency,　intake/exhaust　pressure　loss　and　friction　loss　are　studied.　The　optimal　internal　volume　ratio　and　shaft　efficiency　for　five　working　fluids　(R123,　HFO-1336mzz(Z),　R601,　Cyclopentane,　R245fa)　are　obtained　when　evaporation　temperature　changes　from　373　K　to　463　K.　The　results　show　that　the　optimal　internal　volume　ratio　is　not　the　bigger　the　better　when　SSE　works　at　high　pressure　ratio　condition,　because　intake　pressure　loss　also　increases　with　the　increase　of　internal　volume　ratio.　The　optimal　shaft　efficiency　increases　with　the　decrease　of　friction　power,　but　the　optimal　internal　volume　ratio　is　almost　unaffected.　Furthermore,　reducing　intake　pressure　loss　is　a　powerful　means　to　improve　the　performance　of　SSE　with　large　internal　volume　ratio.　(C)　2019　Elsevier　Ltd.　All　rights　reserved.