Single atom catalysts (SACs) based on 2D materials have been identified to be efficient in many catalytic reactions. In this work, the catalytic performance of Pd/Pt embedded planar carbon nitride (CN) for CO oxidation has been investigated via spin-polarized density functional theory calculations. We find that Pd/Pt can be firmly anchored in the porous CN monolayer due to the strong hybridization between Pd/Pt-d orbitals and adjacent N-2p orbitals. The resulting high adsorption energy and large diffusion barrier of Pd/Pt ensures the remarkable stability of the catalyst Pd/Pt@CN during the CO oxidation reaction. The three distinct CO reaction mechanisms, namely, Eley-Rideal (ER), Langmuir-Hinshelwood (LH), and tri-molecular Eley-Rideal (TER), are taken into consideration comparatively. Intriguingly, the oxidation reaction on Pd@CN prefers to proceed through the less common TER mechanism, where two CO molecules and one O2 molecule need to cross a small reaction barrier of 0.48 eV, and finally dissociate into two CO2 molecules. However, the LH mechanism is the most relevant one on Pt@CN with a rate-limiting reaction barrier of 0.68 eV. Moreover, the origin of the SAC's reactivity enhancement is the electronic "acceptance-donation" interaction caused by orbital hybridization between Pd/Pt and preadsorbed O2/CO. Our findings are expected to widen the catalytic application of carbon-based 2D materials.