At　present,　ducted　air-conditioning　(AC)　unit　is　characterized　of　high　efficiency,　convenient　installation,　fresh　air,　less　initial　investment,　etc.　It　has　been　widely　used　in　restaurant,　supermarket　and　factory　workshop.　This　paper　analyses　advantage　and　disadvantage　of　the　capillary　tube,　single　ported　thermostatic　expansion　valve,　double　ported　thermostatic　expansion　valve.　Especially,　modern　large-scale　buildings　make　indoor　and　outdoor　unit＇　placing　distance　very　far,　how　to　ensure　reliable　running　of　the　system　of　long　tube　is　important,　and　proposed　a　method　that　making　the　balanced　port　thermostatic　expansion　valve　and　capillary　tube　run　parallel　connection,　which　ensuring　reliable　operation　of　the　system　of　long　tube.