As　a　viable　alternative　for　HCFCs　and　HFCs　in　the　refrigeration　and　air　conditioning　industry,　R290　has　obvious　advantages　in　aspects　of　environmental　protection　and　thermal　properties.　At　present,　the　application　of　R290　in　air　source　heat　pumps　(ASHPs)　is　still　in　the　stage　of　exploration　and　promotion.　In　this　paper,　the　dynamic　behaviors　of　an　R290　ASHP　were　experimentally　investigated　during　defrosting　process,　and　an　experimental　platform　of　an　R290　ASHP　system　was　built　to　conduct　defrosting　experiments　in　a　multifunctional　enthalpy-difference　laboratory.　The　variation　of　pressure　and　temperature　of　main　components　and　the　refrigerant　status　in　the　accumulator　during　defrosting　process　are　shown　in　this　paper.　Experimental　data　was　analyzed　by　combining　the　physical　characteristics　of　R290,　and　the　dynamic　characteristics　under　three　different　working　conditions　were　compared.　The　defrosting　time　of　the　three　working　conditions　was　420　s,　435　s　and　865　s　respectively.　In　addition,　there　was　a　point　M　that　the　temperature　difference　between　the　inlet　and　outlet　of　outdoor　heat　exchanger　(ODHE)　approached　zero　at　370　s　of　condition　II.　The　results　and　analysis　of　this　research　can　provide　effective　guidance　for　improving　the　reliability　of　R290　ASHPs.　©　2020　Elsevier　B.V.