In Fig. 2, the flue gas (FLUEGAS) flowing into the absorber from the bottom has already been cooled, desulphurized and denitrified simultaneously by the pretreatment column as shown in Fig. 1. The lean solvent (LEANIN) flows into the absorber from the top. After the lean solvent absorbs the CO2 in the flue gas, it changes to the rich solvent (RICHOUT). After heating by the heat exchanger and the heater, the rich solvent (RICHIN) flows into the stripper from the second stage. In the stripper, the first stage is a condenser and the last stage is a reboiler. The rich solvent is heated and the CO2 is released in the stripper. Then, the rich solvent changes to the lean solvent (LEANOUT). After cooling by the heat exchanger, the lean solvent flows back into the absorber.