Computational　fluid　dynamics　(CFD)　methods　have　been　widely　used　in　simulating　internal　flow　of　rotary　positive　displacement　type　compressors　and　expanders,　especially　for　twin　screw,　scroll　and　rolling　piston　machines.　But　it　is　still　lack　of　CFD　research　on　single　screw　compressors　and　expanders　due　to　the　difficulty　of　spacial　screw　curved　surface　of　single　screw　structure　and　the　working　principle　of　symmetrical　intake　and　six　screw　groove　simultaneous　discharge　in　single　screw　machines.　In　order　to　investigate　internal　unsteady　flow　characteristics　and　improve　the　performance　of　single　screw　machines,　CFD　analysis　in　a　single　screw　expander　has　been　given　in　this　paper.　The　STAR-CCM+　commercial　software　and　its　dynamic　mesh　technology　have　been　applied　to　simulate　the　single　screw　expanders.　The　air　has　been　selected　as　the　working　fluid　of　single　screw　expander　and　the　ideal　gas　equation　of　state　has　been　used　to　analyze　its　properties.　The　simulation　results　show　that　the　periodic　variation　in　pressure　and　velocity　of　single　screw　expander　have　been　carried　out,　the　pressure　cloudy　chart　of　internal　flow　field　of　single　screw　expander　has　been　obtained.　©　Springer　Nature　Switzerland　AG　2020.