Nitrogen-enriched porous carbon fibers were successfully prepared by one-step carbonization-activation treatment of oxidized polyacrylonitrile fibers. Utilizing the low-temperature carbonization and air activation, lots of narrow micropores were developed, and the volume of pores with a size below 0.8 nm was in the range of 0.110-0.122 g/cm(3). Meanwhile, nitrogen preserved in the fibers exceeded 20.8 wt % in content. The porous carbon fiber exhibited an optimal CO2 uptake of 99 mg/g at 25 degrees C and 1 bar. More importantly, an incomparable Henry's law CO2/N-2 selectivity of 183 was achieved, which was primarily attributed to the prominent nitrogen-containing functionalities. Additionally, the sample had a low isosteric heat of adsorption, between 26.65 and 30.84 kJ/mol. This work provides a convenient strategy for the construction of CO2 adsorbent with high selectivity from nitrogen-containing starting materials.