Phosphorus-doped graphene is known to exhibit good electrocatalytic activity for oxygen reduction reaction (ORR). While the ORR activity of P-doped graphene nanoribbons (PGNR) is still unclear. Taking the common graphene nanoribbons with the edges of armchair as an example in this study, we research the mechanistic investigation of ORR on the PGNR under acidic electrolytic conditions by density functional theory (DFT). Based on the keen observation of the atomic charge distribution and adsorption energy at different sites, P atom in PGNR is considered to be the strongest adsorption site with oxygen. Detailed ORR mechanistic was deduced by the investigation of reaction heat, reaction barrier for each possible step and molecular dynamics (MD) simulation. Based on our calculations, when the contribution of the intermediate product to the ORR activity is not considered, PGNR does not possess the property as an ORR catalyst due to several high reaction barriers and some endothermic reactions for ORR path.