Hierarchical porous activated carbon fibers with a BET surface area of 2231 m² g⁻¹ and a pore volume of 1.16 cm³ g⁻¹ were made from polyacrylonitrile through pre-oxidation and chemical activation. This type of material contains a large amount of nitrogen-containing groups (N content > 8.1 wt%) and consequently basic sites, resulting in a faster adsorption rate and a higher adsorption capacity for CO₂ than pure carbon materials with analogous structures under the same conditions. Moreover, its adsorption capacity for CO₂ was more than 3.3-times higher than that for N₂. In particular, it showed a much higher CO₂ adsorption capacity than zeolite 13X, which is conventionally used to capture CO₂, in the presence of H₂O.