Direct reduction of nitrogen oxide waste gas in flue gas (CO x , NO x , etc.) without NH 3 injection is an ideal method to solve nitrogen oxide pollutions. The detailed reaction mechanisms of NO reduction with CO on a single Ni atom based on monolayer g-C 3 N 4 (Ni−g-C 3 N 4 ), such as Eley−Rideal, Langmuir−Hinshelwood, and termolecular Eley−Rideal, were investigated by first-principles calculation. The results indicated that Ni−g-C 3 N 4 had excellent NO adsorption capacity and could be an active NO adsorption removal material. The termolecular Eley−Rideal was the fundamental reaction mechanism for NO reduction with a rate-determining barrier of 0.19 eV based on the thermodynamic and kinetic analysis. The intermediate N 2 O could be reduced to N 2 rapidly on Ni−g-C 3 N 4 with a rate-determining barrier of 0.50 eV, and the CO in the flue gas would rapidly regenerate the oxidized Ni−g-C 3 N 4 . This work predicted a novel efficient catalyst Ni−g-C 3 N 4 for NO reduction to N 2 with CO combining with experimental verification and provided a new insight for NO removal.