To　achieve　the　goal　of　clean　annual　space　heating　and　cooling　for　residential　buildings,　the　concept　of　annual　space　heating　and　cooling　transcritical　CO2　system　combined　with　ejector　and　mechanical　subcooling　is　proposed.　The　problem　of　large　throttling　loss　and　performance　degradation　when　CO2　system　is　applied　for　residential　space　heating　and　cooling　can　be　solved.　A　theoretical　model　of　the　new　proposed　system　is　developed,　and　then　optimized　by　the　method　of　genetic　algorithm.　Moreover,　taking　an　ordinary　residential　building　located　in　70　cities　belonging　to　different　climate　zones　as　the　application　scenario,　the　seasonal,　annual,　environmental,　and　economic　performance　of　eight　heating　and　cooling　solutions　are　compared　and　discussed.　The　results　indicate　that　the　COP　and　exergy　efficiency　of　the　new　proposed　system　is　significantly　improved　in　contrast　to　conventional　mechanical　subcooling　system,　which　increases　by　11.48%　and　21.64%　in　heating　mode,　and　8.99%　and　8.69%　in　cooling　mode,　respectively.　The　annual　performance　coefficient　of　the　new　proposed　system　is　7.21%∼8.21%　higher　than　conventional　mechanical　subcooling　system.　Additionally,　the　life　cycle　carbon　emissions　of　the　new　proposed　system　can　be　significantly　reduced　by　12.23%∼89.58%　compared　with　the　combined　solution　of　coal-fired　boiler　and　R32.　With　the　advancement　of　CO2　technology,　it　is　possible　to　improve　the　economic　performance　of　CO2　systems　and　the　price　of　the　new　proposed　system　is　lower　than　all　solutions　when　the　price　of　compressor　is　reduced　by　60%.　This　study　can　provide　a　theoretical　reference　for　the　development　and　optimization　of　CO2　combined　heating　and　cooling　system,　and　provide　a　feasible　solution　for　residential　annual　space　heating　and　cooling.　©　2023　Elsevier　Ltd