Metal-free carbon-based nanomaterials have been widely studied as a type of excellent electrocatalyst for the oxygen reduction reaction (ORR) in an alkaline medium due to their great stability, wide availability, environmental acceptability, and strong resistance to poisonous gas. They usually show a much inferior activity in an acidic medium, and a strategy to modify sp² carbon to facilitate the ORR in an acidic medium is critically needed. Herein, by taking carbon nanotubes (CNT) as the platform, and O₂ chemisorption ability as the descriptor of activity in an acidic medium, a series of N, B, BO_x, P and S mono- and multi-doped CNT(5,5) were studied using an ab initio modeling method. Eight configurations resulting from N mono-doping, BO_x doping and B/N, BO_x/N and P/N multi-doping are found to be potential active structures in an acidic medium due to their highly exothermic O₂ chemisorption. These results indicate the great potential of sp² carbon as an ORR electrocatalyst in an acidic medium, and also provide theoretical references for the exploration of carbon-based metal-free ORR electrocatalysts.