The design and development of non-noble metal carbon-based catalysts with efficient oxygen reduction activity remains a difficult problem. Here, Ultrathin two-dimensional carbon nano sheet catalyst Fe/Mn-SNC with rich pore structure doped with trace manganese was synthesized by simple sodium chloride template cracking method and freeze-drying method. Citric acid containing a large number of OH and COOH groups can provide multiple coordination sites for metal ions. In 0.1 M KOH solution, the Fe/Mn-SNC catalyst performed better than Pt/C in terms of ORR performance (E1/2 was 0.920 V vs RHE) and endurance (E1/2 fell roughly 5 mV after 10,000 cycles). The reaction path of the Fe/Mn-SNC was calculated through DFT calculation. It was found that a small amount of Mn doping would reduce the adsorption energy of OH*, the last step in ORR reaction, which was conducive to OH* falling off from the catalyst surface, reducing the potential barrier required for the reaction, and improving its ORR activity. This research offers fresh perspectives on the development of enhanced energy conversion as well as the design and synthesis of high-performance ORR catalysts.