Lithium-oxygen battery possesses an extremely high theoretical energy density ( \approx 3500 W·h·kg^–1), and is an ideal next-generation energy storage system. The ideal operation of lithium-oxygen batteries is based on the electrochemical formation (discharge) and decomposition (charge) of lithium peroxide (Li₂O₂). At the beginning of the discharge, oxygen is reduced on the electrode, forming an oxygen radical (\rm O^-_2 ). The \rm O^-_2 successively combines with an Li ion, forming the metastable LiO₂. The LiO₂ may subsequently undergo two different reaction pathways: a chemical disproportionation and a continuous electrochemical reduction, thereby resulting in the formation of Li₂O₂. Therefore, the oxygen reduction reaction (ORR) is an important step in the discharge process. Studies have shown that graphene is considered as the most promising cathode material for non-aqueous lithium-oxygen batteries. Moreover, it is found that nitrogen-doped graphene has higher electrocatalytic activity than intrinsic graphene for the ORR. However, up to now, the mechanism of improving the ORR for nitrogen-doped graphene is still unclear, and the effects of different N-doping concentrations on the ORR have not been reported. In this work, on the basis of the first-principles calculations, the reduction mechanism of O₂ molecule by nitrogen-doped graphene with different N concentrations is studied. Results show that after doping N atoms, the adsorption energy of O₂ molecules increases, the O—O bond length is elongated, and the transferred charge increases, which indicates that nitrogen-doped graphene enhances the reduction ability of O₂ molecule. Bader charge analysis shows that both N atom and O₂ molecule obtain charges from C atom, and N atom also provides charges for O₂ molecule, which is consistent with the electronegativity of carbon, nitrogen and oxygen. This charge transfer results in the stronger interaction between the O₂ molecule and the substrate, and can reveal the reason why nitrogen-doped graphene can improve the ORR. In addition, it is found that the reduction ability of O₂ molecule is best when the N-doping ratio is 3.13 at%. It is hoped that this work will play a guiding role in the synthesizing the nitrogen-doped graphene materials, and will be helpful in optimizing the cathode materials of lithium-oxygen batteries.