To　know　energy　efficiency　utilization　and　obtain　a　high　capacity　usage　ratio　for　the　air　power　system　based　on　single-screw　engine,　the　energy　loss　for　each　process　and　the　total　output　work　are　analyzed,　also　the　influence　of　exhaust　pressure　on　energy　distribution　is　studied　emphatically　by　taking　energy　analysis　method　and　thermodynamic　theory　concerning　non-steady　flow　with　variable　mass　system.　The　results　indicate　that　adopting　low　exhaust　pressure　is　good　for　increasing　the　mass　and　energy　utilization　efficiency　of　the　power　system.　The　mass　utilization　ratio　is　more　than　90　percent　while　energy　efficiency　is　low,　which　is　due　to　great　energy　losses　during　decompression　and　exhaust　process.　Where,　the　energy　loss　produced　during　decompression　process　takes　more　than　half　of　the　total　energy.　Therefore,　how　to　reduce　the　energy　loss　during　the　said　two　main　processes　is　the　key　point　to　increase　energy　utilization　of　the　power　system,　especially　for　the　decompression　process.　©　2011　Journal　of　Mechanical　Engineering.