When　the　air-source　heat　pump　system　is　operated　under　low　temperature　conditions,　there　exists　some　problems　such　as　frost　formation　on　the　evaporator　and　deterioration　of　system　heating　performance.　Considering　the　limitations　of　traditional　defrosting　methods　applied　in　the　transcritical　CO2　heat　pump　system,　the　hot-gas　bypass　defrosting　method　was　experimentally　investigated.　The　platform　of　air-source　transcritical　CO2　heat　pump　system　was　designed　and　a　copper　bypass　tube　with　an　outer　diameter　of　12.7　mm　was　used　as　the　expansion　device.　The　platform　was　tested　under　various　conditions　to　analyze　the　dynamic　parameters　during　defrosting　process　and　the　effect　of　ambient　temperature　on　the　defrosting　time.　Meanwhile,　the　defrosting　process　at　different　times　was　depicted　in　the　pressure-enthalpy　diagram.　The　experimental　results　show　that　the　hot-gas　bypass　defrosting　process　is　relatively　stable,　and　the　parameters　of　each　measuring　point　change　relatively　gently.　According　to　the　experimental　data,　it　can　be　found　that　the　hot-gas　bypass　defrosting　method　can　significantly　increase　the　evaporator　inlet　temperature　to　about　30,　effectively　shortening　the　defrosting　time.　The　defrosting　time　was　greatly　affected　by　the　defrosting　stability　period.　The　decrease　of　environmental　temperature　or　the　increase　of　environmental　humidity　would　extend　the　defrosting　time　of　the　system.　The　change　trend　of　defrosting　energy　consumption　ratio　is　basically　consistent　with　the　defrosting　time　ratio.　The　defrosting　efficiency　is　calculated　to　be　46.5%　and　33.62%　higher　than　that　of　other　defrosting　methods　and　the　defrosting　time　is　shortened　by　100　s,　which　indicates　that　the　hot-gas　bypass　defrosting　method　is　more　suitable　for　the　air-source　transcritical　CO2　heat　pump.　©　2019,　Shanghai　Jiao　Tong　University　Press.　All　right　reserved.