To　improve　the　efficiency　of　air　source　heat　pumps　and　eliminate　the　performance　degradation　caused　by　frost　in　winter,　a　hot　gas　bypass　defrosting　experimental　platform　was　built　and　the　hot　gas　bypass　defrosting　experiment　under　different　outdoor　temperature　and　different　opening　degree　of　hot　gas　bypass　valve　were　carried　out.　The　results　showed　that　the　defrosting　of　the　heat　pump　evaporator　can　be　completed　well　by　adopting　the　hot　gas　bypass　defrosting　method　without　interrupting　the　heat　pump　heating.　At　outdoor　temperatures　of　-10　degrees　C,　-5　degrees　C　and　0　degrees　C,　the　most　appropriate　time　for　heat　pump　to　activate　hot　gas　bypass　defrosting　were　30　min,　40　min　and　50　min,　respectively.　The　corresponding　peak　value　of　average　heating　capacity　were　7.96　kW,　10.16　kW,　and　12.08　kW,　and　the　heating　COPC　values　were　3.03,　3.56,　and　4.19,　respectively.　The　increase　of　bypass　exhaust　volume　would　shorten　the　defrosting　time　but　also　reduce　the　heating　capacity　of　the　heat　pump　during　the　defrosting　process.　Hence　the　optimum　defrosting　start　time　of　the　system　is　different　with　different　bypass　valve　opening.　For　the　system　built　by　this　paper,　the　20　%　bypass　valve　opening　had　the　best　performance　in　defrosting,　and　the　optimal　defrosting　start　time　was　40　min,　during　which　the　average　heating　capacity　and　COPC　achieved　peak　values　of　10.16　kW　and　3.56,　respectively.　These　results　provide　a　reference　for　the　defrosting　design　of　air　source　heat　pumps.