Due　to　its　excellent　thermodynamic　properties,　low　GWP　refrigerant　R290　is　found　to　be　promising　alternative　refrigerant.　However,　due　to　its　special　physical　properties,　high　oil　discharge　rate　and　throttle　valve　blockage　may　occur　for　the　R290　heat　pump　system　under　low　temperature　start-up　conditions.　In　this　paper,　the　transient　characteristics,　including　the　temperature,　heat　exchange　rate,　phase　change　rate,　oil　discharge　rate　and　the　liquid　level　of　the　oil　sump,　of　a　R290　rotary　compressor　used　in　the　heat　pump　system　under　low　temperature　conditions　are　theoretically　and　experimentally　studied.　A　transient　heat　and　mass　transfer　model　in　the　compressor　is　proposed,　which　considers　the　phase　change　of　refrigerant　in　the　shell.　The　experiment　was　carried　out　on　the　start-up　characteristics　of　the　compressor　and　the　mathematical　model　was　verified.　The　results　show　that　high　rotation　speed　and　low　temperature　are　the　main　reasons　for　the　condensation　of　R290　in　the　shell.　The　excessive　liquid　refrigerant　in　the　shell　leads　to　a　high　oil　discharge　rate　of　up　to　22　%.　This　paper　will　provide　guidance　for　the　use　of　R290　refrigerant　under　low　temperature　conditions.