During　reverse-cycle　defrosting　of　an　air-source　heat　pump,　the　defrosting　time　is　always　long　owing　to　insufficient　heat　sources.　For　the　quasi-two　stage　compression　air-source　heat　pump　(QTASHP),　owing　to　its　larger　outdoor　coil　operated　at　lower　ambient　temperatures,　a　considerable　amount　of　heat　is　dissipated　from　the　surface　of　the　outdoor　coil　during　defrosting,　such　that　the　long　defrosting　time　problem　becomes　more　severe.　To　improve　the　defrosting　performance　of　the　QTASHP,　defrosting　with　vapor　injection　was　proposed　based　on　the　existing　cycle　structure.　Five　groups　of　experiments　were　designed　to　investigate　the　defrosting　performance.　The　results　showed　that　vapor　injection　was　beneficial　for　shortening　the　defrosting　duration,　decreasing　power　consumption,　and　increasing　defrosting　efficiency.　At　the　optimal　opening　of　the　injection　electronic　expansion　valve,　the　defrosting　duration　was　shortened　by　20.61%;　the　heat　absorbed　from　water　and　power　consumption　decreased　by　8.74%　and　17.98%,　respectively;　whereas　the　defrosting　efficiency　increased　by　6.22%.　©　2023　Journal　of　Refrigeration.　All　rights　reserved.