During　the　defrosting　process　of　an　air　source　heat　pump　with　vapor　injection　(ASHPVI),　the　defrosting　duration　lasts　rather　long　due　to　shortage　of　heat　sources　for　defrost.　In　theory,　vapor　injection　during　defrosting　can　shorten　defrosting　duration　due　to　the　increase　of　discharge　refrigerant　and　power　input　of　compressor.　However,　the　injection　electronic　expansion　valve　is　always　turned　off　during　defrosting　because　vapor　injection　technology　is　mainly　used　for　heating　mode.　To　shorten　the　defrosting　duration　of　ASHPVIs,　a　test　rig　using　a　multi-split　air　source　heat　pump　with　vapor　injection　was　established.　Its　defrosting　performance　was　investigated　through　experiment　and　theoretical　analysis.　Results　showed　that　the　injection　ratio　was　0-10.08%　during　defrosting　experiment,　which　shortened　the　defrosting　duration　by　7.75%.　The　defrosting　efficiency　reached　up　to　54.11%　at　the　outdoor　temperature　of　-7.7　degrees　C.　In　addition,　the　effect　of　vapor　injection　on　defrosting　speed　was　mainly　at　the　evaporating　melted　frost　stage.　The　results　of　preliminary　experiments　showed　a　little　refrigerant　could　be　injected　into　compressor　at　defrosting　initial　stage　and　melting　frost　stage,　but　the　amount　of　vapor　injection　increased　obviously　and　accelerated　defrosting　speed　at　evaporating　melted　frost　stage　owing　to　high　injection　pressure　difference.　(C)　2020　Elsevier　B.V.　All　rights　reserved.