Nowadays,　the　main　two　issues　of　heating　ventilation　and　air　conditioning　(HVAC)　are　refrigerant　alternative　and　system　efficiency　improvement.　As　a　safe　and　environmental　protective　refrigerant,　CO2　has　drawn　a　lot　of　attention　these　years.　The　improvement　on　its　transcritical　cycle　efficiency　has　always　been　the　study　hotspot.　On　the　basis　of　the　traditional　cycle　system,　this　paper　adds　the　internal　heat　exchangers　(IHE)　and　pressure　control　devices　into　the　system　and　optimizes　the　CO2　transcritical　system　with　the　dual　throttling　device.　Engineering　equation　solver　is　used　to　build　each　module　in　the　system　as　an　object-oriented　model.　The　effects　of　the　IHE　and　bypass　valve　on　system　performance　are　investigated.　The　performance　and　configuration　of　four　CO2　transcritical　refrigeration　cycles　are　analyzed　and　compared.　Numerical　studies　also　propose　the　formula　expressing　the　relationship　of　the　optimum　high　pressure,　the　gas-cooler　CO2　outlet　temperature　and　the　evaporation　temperature　under　specific　operating　conditions.　In　addition,　the　modified　system　can　significantly　improve　the　performance　of　the　system　under　the　operating　conditions　below　the　optimum　high　pressure.　This　research　has　played　an　effective　guiding　and　facilitating　role　for　CO2　transcritical　refrigeration　system　development,　optimization　and　improvement.　(C)　2017　Institution　of　Chemical　Engineers.　Published　by　Elsevier　B.V.　All　rights　reserved.