The development of carbon dioxide (CO2) capture technology is of great urgency for reducing the emission of CO2 to the atmosphere and mitigating global warming. Polyacrylonitrile-based electrospun carbon nanofibers were prepared in this study at different carbonization temperatures for CO2 capture. The effects of the primary surface features and the functional groups of the carbon nanofibers on CO2 adsorption capacity were discussed. Results showed that the carbonization temperature influenced the porous texture and the surface chemical states of the carbon nanofibers significantly. The specific surface area, total pore volume, and micropore volume of the fibers increased with increasing carbonization temperature, but the ultra-micropore volume presented a different trend. The samples carbonized at 750 °C had the smallest average pore hydraulic radius. Moreover, in the micropore range, the volume of sub-micropores increased at a greater rate than that of ultra-micropores after activation. The carbonization temperature was also of great importance in controlling the nitrogen content and composition. A CO2 uptake of 3.47 mmol/g at 25 °C and 1 atm was achieved. The ultra-micropore volume of the carbon nanofibers was the most important parameter for determining CO2 uptake at 1 atm; however, the CO2 adsorption capacity at 0.15 atm was highly dependent on the surface pyrrolic or pyridonic groups.