The CO2 emitted by coal-fired power plants is the main factor leading to global warming, and the capture of CO2 in the flue gas of power plants is still the main task at this stage. Many adsorbents have been developed to capture CO2 in high-temperature flue gas, but some materials are complicated to synthesize or the cost is too high. Here, we used low-cost raw materials activated carbon fiber and PEI, and used green synthesis to synthesize new adsorbents in order to capture CO2 in high-temperature flue gas of a power plant. To improve the performance of highly porous activated carbon fiber (ACF) in CO2 capture and separation, an organic polymer polyethylenimine (PEI) was loaded successfully into the oxidized ACF. The modified adsorbent was tested by FT-IR, XRD and SEM, and the CO2 adsorption capacity and CO2/N2 selectivity were analyzed. The results showed that the as-synthesized PEI-modified adsorbent has a CO2 adsorption capacity of 2.5mmol/g, which is 1.7 times better than that of the pristine ACF adsorbent (1.5mmol/g), at 1 bar and 333 K, and it has excellent CO2/N2 selectivity, as calculated by ideal adsorption solution theory (IAST). These data indicate that PEI was loaded successfully into the oxidized ACF. In addition, the dual site Langmuir isotherm equation and Langmuir isotherm equation can be in good agreement with the adsorption curves of CO2 and N2. In comparison with other composite adsorbents, the preparation process of the present new adsorbent is highly environmentally friendly, the synthesis method is simple and the cost is low, which demonstrates potential applications in the separation of CO2 from the flue gas of power plants.