Recently, capture and storage of carbon dioxide (CO2) has been an effective strategy to address global warming caused by excess CO2. Porous boron nitride (BN) with tunable porosity, excellent chemical stability and thermal stability have been considered as promising materials for CO2 adsorption. Here, porous BN fibers with high specific surface area have been used as promising adsorbent for CO2 capture. The effects of high-temperature treatments in different gaseous ambient on the microstructure evolution and CO2 adsorption capacity of porous BN fibers are studied in detail. Interestingly, the porous BN fibers after high-temperature post-treatment in NH3 gas show enhanced CO2 adsorption capacity, as compared with pristine porous BN and nearly three times more than porous BN nanosheets reported before. The NH3-treatment can effectively restrain the damage of pore structure caused by high-temperature annealing, especially micropores. Moreover, in the NH3-treatment process, the impurities (B-O) of porous BN fibers can react with NH3 to form B-N-H, which is the root cause of the enhancement of CO2 adsorption capacity of porous BN fibers.