Due to the depletion of the ozone layer and the global climate warming,CO_2 is promoted and applied extensively in the transcritical CO_2 system as a new type of refrigerant.Adding the internal heat exchanger in the loop can sharply improve the performance of the system.This paper deeply discussed the specific effects on the performance of the system in the aspects of the performance of transcritical CO_2 system added with the internal heat exchanger,the structure of the internal heat exchanger,the location of the internal heat exchanger in the loop,the efficiency of the internal heat exchanger and the temperature of the refrigerant at inlet of the internal heat exchanger for the application of the internal heat exchanger in the transcritical CO_2 system.In addition,the progress in researches at home and abroad was introduced and the application of the internal heat exchanger was analyzed and prospected.