The　dynamic　characteristics　for　both　the　hot-gas　bypass　defrosting　method　and　the　reverse-cycle　defrosting　method　were　compared　experimentally　on　a　55　kW　unitary　air-to-water　heat　pump.　The　duration　time　for　the　reverse-cycle　defrosting　method　was　94　seconds　because　the　two　energy　sources　for　the　melting　frost　were　the　input　power　of　the　compressor　and　the　heat　absorbed　from　the　circulating　water　and　indoor　space.　But　the　indoor　room　temperature　decreased　greatly　and　quickly,　which　destroyed　the　indoor　amenity.　The　duration　time　for　the　hot-gas　bypass　defrosting　method　was　178　seconds　more　than　that　for　the　reverse-cycle　defrosting　method　because　the　only　energy　source　was　the　input　power　of　the　compressor.　Moreover,　the　larger　heat　loss　caused　by　the　distributor　and　distributor　capillary　was　the　other　main　cause　of　the　longer　defrosting　time　for　the　hot-gas　bypass　defrosting　method.　Since　the　heat　was　not　absorbed　from　the　indoor　space,　the　amenity　was　greatly　improved.　However,　the　discharge　temperature　and　superheat　decreased　gradually　because　the　compressor　suction　superheat　was　always　0°C　during　all　the　melting　stage　for　the　hot-gas　bypass　defrosting　method.　Correspondingly,　we　must　prevent　the　discharge　temperature　and　superheat　from　decreasing　to　a　level　where　the　compressor　does　not　function　safely.