Abstract: The present disclosure discloses the concept and expression method of an energy efficiency index (EEI) COPCO2 for a carbon-capture system. The COPCO2 refers to the ratio of the gain during a carbon-capture process to the cost practically, and to the ratio of the increase of CO2 chemical potential resulting from enrichment to the driving work input into the carbon-capture system physically. By introducing the concept of environmental effective state (G state), the present disclosure quantifies the “gain” during a carbon-capture process, and compares the “gain” with the “cost” to obtain the COPCO2 expression method of coefficient of performance (COP) during the carbon-capture process, which provides more accurate evaluation for the energy utilization level during a carbon-capture process and is conducive to improving the energy utilization level by carbon-capture technology, thereby improving the energy utilization rate.