Single-atom catalysts, especially with single Pt atoms, have attracted more and more attention due to their high catalytic activity for CO oxidation. The outstanding stability and catalytic activity of a single Pt atom supported on nitrogen doped graphene (Pt/NG) are revealed using first-principles calculations. We find that the stability of a Pt atom on the NG can be promoted by picking an appropriate doping configuration. The exceptionally stable Pt/NG catalyst exhibits excellent catalytic activity for CO oxidation via a new tri-molecular Eley–Rideal mechanism (2CO + O₂ → OCO–OCO → 2CO₂) with an energy barrier of 0.16 eV for the rate-limiting step of OCO–OCO dissociation, which is more preferable than the other two normal Langmuir–Hinshelwood and Eley–Rideal mechanisms.