Single-atom catalysts (SACs) often present outstanding activity due to their high ratio of low-coordinated metal atoms and can be applied to the activation of strong chemical bonds such as C[triple bond, length as m-dash]O. Herein, we investigate the potential usage of a single-atom catalyst, in which isolated cobalt atoms are supported on porous graphitic carbon nitride (Co/g-C3N4), for CO oxidation. Based on the adsorption/co-adsorption energies of O2, CO, 2O2, CO + O2 and 2CO, the screening criteria and the reaction mechanisms of CO oxidation, including the Eley-Rideal, New Eley-Rideal, Langmuir-Hinshelwood, and termolecular Eley-Rideal mechanisms, are established and compared. In particular, the energy barriers of the rate-limiting steps for the CO oxidation process by all possible reaction pathways are in a range from 0.21 to 0.59 eV, suggesting that the Co/g-C3N4 catalyst can boost CO oxidation at low temperature. Moreover, the preparation of the SAC (Co/g-C3N4) by using CoCl2 as an appropriate metal precursor and the stability (up to 600 K) are evaluated by ab initio molecular dynamics simulations. The high stability and excellent activity of the Co/g-C3N4 SAC for CO oxidation offer a high possibility of clean energy production.