In this review, the research progress of compressor, evaporator and air cooler in trans-critical CO₂ refrigeration system was analyzed. It was concluded that the method of improving the refrigeration efficiency and the optimum heat transfer efficiency of the trans-critical CO₂ refrigeration system was obtained. The application of trans-critical CO₂ refrigeration cycle in CO₂ automotive air conditioning, heat pump and food processing were also discussed. The problems of high system pressure in CO₂ refrigeration cycle in heat pump were analyzed. Finally, it was proposed that the development of oil-free compressor in trans-critical CO₂ refrigeration cycle was one of the main research directions in the future. The application of CO₂ refrigeration system in steaming and freezing of aquatic products, slaughtering of meat and refrigerating transportation of ice cream was prospected.