Currently,　the　alternative　refrigerant　in　air　conditioning　systems　has　become　a　global　problem,　and　air　as　a　natural　working　fluid　has　several　advantages　of　zero　ODP,　low　GWP　and　satisfactory　thermodynamic　properties,　thus　it　is　a　long-term　development.　However,　the　poor　performance　of　reverse　Brayton　air　cycle　limits　their　application,　and　how　to　improve　the　performance　is　the　main　problem　we　must　face　at　present.　Thus,　enhancing　the　components＇　efficiency　and　improving　the　system　comprehensive　performance　have　become　the　most　effective　way.　The　output　power　produced　by　the　expander　can　be　used　for　coaxial　compressors,　such　as　tur-boexpander　compressor　(TEC),　turbocharger,　and　rotary　vane　compressor-expander　(ROVCE).　What＇s　more,　amounts　of　new　technological　approaches　have　been　used　to　improve　the　performance　of　the　systems　such　as　regeneration,　multi-stage　compression,　mixed　working　fluid,　water　removal　and　integrated　system.　In　addition,　thermodynamic　optimization　methods　have　become　one　of　the　most　important　approaches　to　reduce　the　irre-versible　losses　and　enhance　the　system　performance.　At　present,　reverse　Brayton　air　cycle　machines　have　been　applied　in　various　fields　such　as　food　freezing,　transportation,　heat　pump,　refrigerator,　and　dryer,　etc.　With　the　increasingly　stringent　environmental　laws　and　regulations,　the　air　cycle　system　will　have　a　wide　perspective　application　in　the　future.