The　transcritical　CO2　heat　pump　is　widely　used　due　to　its　higher　efficiency.　Frost　is　inevitable　at　low　ambient　temperature,　resulting　in　a　deteriorated　coefficient　of　performance　and　heating　capacity.　Considering　the　disadvantages　and　limitations　of　other　defrosting　methods,　the　hot　gas　bypass　defrosting　method　was　experimentally　investigated　in　this　paper.　The　dynamic　characteristics　of　pressure　and　temperature　during　the　defrosting　process　are　studied　in　detail　and　the　pressure-enthalpy　diagrams　are　depicted.　In　addition,　energy　analysis　of　the　heat　supply　and　energy　consumption　is　presented　to　evaluate　the　defrosting　efficiency.　Furthermore,　the　influence　of　ambient　temperature　is　also　discussed.　The　results　show　that　the　measured　parameters　change　gradually　during　the　defrosting　process　except　for　the　initial　stage　and　defrosting　time　is　within　the　defrosting　regulations.　The　defrosting　efficiency　is　evaluated　to　be　41.16%–50.84%　with　variation　of　ambient　temperatures,　which　indicates　that　the　hot　gas　bypass　defrosting　method　is　suitable　for　the　air　source　transcritical　CO2　heat　pump.　©　2020　Elsevier　Ltd