Cost-effective biomass-derived activated carbons with a high CO2 adsorption capacity are attractive for carbon capture. Bamboo was found to be a suitable precursor for activated carbon preparation through KOH activation. The bamboo size in the range of 10–200 mesh had little effect on CO2 adsorption, whereas the KOH/C mass ratio and activation temperature had a significant impact on CO2 adsorption. The bamboo-derived activated carbon had a high adsorption capacity and excellent selectivity for CO2, and also the adsorption process was highly reversible. The adsorbed amount of CO2 on the granular activated carbon was up to 7.0 mmol g−1 at 273 K and 1 bar, which was higher than almost all carbon materials. The pore characteristics of activated carbons responsible for high CO2 adsorption were fully investigated. Based on the analysis of narrow micropore size distribution of several activated carbons prepared under different conditions, a more accurate micropore range contributing to CO2 adsorption was proposed. The volume of micropores in the range of 0.33–0.82 nm had a good linear relationship with CO2 adsorption at 273 K and 1 bar, and the narrow micropores of about 0.55 nm produced the major contribution, which could be used to evaluate CO2 adsorption on activated carbons.