As　a　new　generation　of　heat　pump　refrigerant,　R290　(propane)　not　only　has　advantages　in　environmental　protection,　but　also　has　the　good　performance.　However,　reliability　problems　remain　in　the　start-up　process　of　R290　heat　pumps,　and　hence　it　is　necessary　to　study　the　dynamic　characteristics　of　the　start-up　process.　This　paper　establishes　a　mathematical　model　based　on　the　physical　properties　of　R290　and　the　system　characteristics　of　R290　split　heat　pumps.　The　model　is　used　to　calculate　the　dynamic　start-up　characteristics.　The　proposed　mathematical　model　is　validated　by　experiments,　and　an　error　analysis　of　the　heating　start-up　is　carried　out,　and　the　mechanism　of　continuous　low　suction　pressure　during　start-up　is　analysed.　Taking　a　variable　frequency　rated　heating　condition　as　an　example　(indoor　and　outdoor　temperature　20/7　℃),　the　maximum　deviation　between　the　calculated　pressure　and　the　experimental　value　is　within　5%　during　stable　process.　The　refrigerant　mass　in　the　compressor　shell　is　164.76　g　at　most　and　the　liquid　separation　process　lasts　370　s　during　heating　start-up　process.　The　continuous　suction　of　low　pressure　in　the　R290　split　heat　pump　is　attributed　to　the　accumulation　of　liquid　phase　refrigerants　in　the　accumulator　and　the　presence　of　refrigerant　condensation　in　the　compressor　shell.　©　2023