Using　high-performance　expanders　is　one　of　the　keys　to　improve　the　efficiency　of　Organic　Rankine　Cycle　(ORC).　However,　in　large　expansion　ratio　conditions,　the　performance　deterioration　caused　by　under-expansion　seriously　affects　the　performances　of　the　ORC.　For　single-screw　expanders,　it　is　possible　to　utilize　the　kinetic　energy　of　the　screw　groove　exhaust　to　produce　extra　power.　But　there　is　still　a　lack　of　theoretical　support　for　optimizing　the　exhaust　kinetic　energy　utilization　process　of　this　new　expander　at　present.　Therefore,　the　energy　conversion　mechanism　of　Single　Screw　Expander　with　exhaust　Kinetic　energy　Utilization　(SSEKU)　was　investigated　in　this　paper,　and　a　mathematical　model　of　SSEKU　was　developed　to　investigate　the　energy　conversion　mechanisms　and　predict　the　performances.　The　results　show　that　introducing　exhaust　kinetic　energy　utilization　process　in　single　screw　expander　can　reduce　under-expansion　losses　effectively,　and　the　effects　in　large　size　expanders　were　stronger　than　that　in　small　scale　machines.　It　was　also　found　that　the　maximum　increase　in　isentropic　efficiency　was　3.01,　5.47,　and　9.38　percentage　points　for　the　expander　with　a　screw　diameter　of　117　mm,　200　mm,　and　350　mm,　respectively.