When　CO2　heat　pump　is　used　for　space　heating,　the　high　return　water　temperature　leads　to　excessive　throttling　loss　and　low　efficiency　of　the　heat　pump　unit.　In　order　to　solve　this　problem,　an　air-to-water　CO2　heat　pump　unit　combined　with　phase　change　thermal　storage　for　space　heating　was　proposed　and　its　feasibility　has　been　analyzed.　On　the　basis　of　previous　research,　a　simulation　analysis　was　conducted　in　this　work　to　further　analyze　and　optimize　the　performance　of　the　combined　system.　Firstly,　a　mathematical　model　of　the　combined　system　was　established,　and　the　correctness　of　the　model　was　verified　by　experimental　results.　Secondly,　the　influence　of　the　key　parameters,　such　as　the　ambient　temperature,　the　inlet　water　temperature　of　gas　cooler,　on　the　system　performance　was　discussed,　and　a　correlation　of　the　optimal　discharge　pressure　for　the　system　was　developed.　The　results　indicated　that　the　COP　of　CO2　heat　pump　unit　increased　by　13.3%　when　the　ambient　temperature　rose　from　-23　degrees　C　to　-5　degrees　C　at　the　inlet　water　temperature　of　30　degrees　C.　Lastly,　an　optimized　control　strategy　of　the　combined　system　was　developed　under　the　different　operating　modes.　These　results　provided　a　basis　in　improving　and　optimizing　the　performance　of　the　combined　system　and　helping　to　promote　its　application.　(C)　2019　Elsevier　B.V.　All　rights　reserved.