For　air　source　heat　pump　with　vapor　injection　(ASHPVI),　the　vapor　injection　can　improve　its　heating　performance　of　ASHPVI　at　non-frosting　low　temperature　condition.　However,　its　effect　under　the　frosting　condition　is　still　unclear.　To　improve　the　heating　performance　of　ASHPVI　under　frosting　condition,　a　test　rig　for　the　frosting　of　ASHPVI　was　built　in　a　psychrometer　testing　room.　Then,　a　new　control　strategy　for　vapor　injection　was　proposed　to　ensure　its　safety,　and　two　groups　of　frosting　experiments　with　and　without　vapor　injection　were　conducted.　Results　showed　that　under　frosting　condition,　vapor　injection　can　still　improve　the　heating　capacity　of　ASHPVI　by　11.31　%.　It　can　also　make　the　outdoor　coil　surface　temperature　decreased　by　0.8-4.6　degrees　C,　and　further　result　in　a　faster　frosting　speed.　Compared　with　the　frosting　duration　without　vapor　injection　(64　min),　the　frosting　duration　with　vapor　injection　is　shortened　by　14　min.　Owing　to　the　faster　frosting　speed,　the　average　COP　with　vapor　injection　decreases　by　0.17,　with　a　decrease　rate　of　6.03　%.　When　the　heating　capacity　of　AHSPVI　without　vapor　injection　under　frosting　condition　can　meet　the　building　heating　load,　the　vapor　injection　should　not　be　adopted.