During　the　defrosting　process　in　cold　regions,　the　air　source　heat　pump　with　vapor　injection　encounters　some　problems,　e.g.　long　defrosting　time　and　low　defrosting　efficiency.　To　solve　these　problems,　a　new　defrosting　strategy　through　injecting　medium-pressure　vapor　refrigerant　into　compressor　during　defrosting　process　was　proposed　based　on　its　special　structure.　Five　groups　of　experiments　having　different　openings　of　injection　electronic　expansion　valve　were　designed　and　conducted　to　investigate　the　effect　of　this　defrosting　strategy.　Results　indicate　that　an　optimal　opening　of　injection　electronic　expansion　valve　exists　during　defrosting　process　with　vapor　injection.　For　the　experimental　air　source　heat　pump,　it　is　50%.　At　the　opening　of　less　than　50%,　the　experimental　unit　can　run　safely　and　defrosting　performance　improves　gradually　with　the　opening　increase.　When　the　opening　increases　to　greater　than　50%,　the　defrosting　process　is　unsteady.　At　the　optimal　opening　of　injection　electronic　expansion　valve,　the　defrosting　time　and　power　consumption　are　respectively　decreased　by　20.61%　and　17.98%,　while　the　defrosting　efficiency　is　improved　by　6.22%.　©　2021　Elsevier　B.V.