An experimental and numerical study of the NO x formation and reduction process in a designed coal combustion furnace under both traditional air atmosphere and O2/CO2 atmosphere was conducted, in an attempt to explore the chemistry mechanism of the experimentally observed NO x suppression under high CO2 concentration atmospheres. A simplified’ chemically oriented’ approach, computational fluid dynamics (CFD)-chemical kinetics modeling method, was validated and used to model the experimental process. The high CO2 concentration’s chemical effect on NO reduction has been studied, and the differences in NO x reaction behaviors between O2/CO2 atmosphere and air atmosphere were analyzed by detailed chemical kinetic model. On the basis of investigations through elementary chemical reactions, it can be concluded that high CO2 concentration plays an important role on NO x conversion process during oxy-fuel combustion. Moreover, the dominant reaction steps and the most important reactions for NO conversion under different atmospheres were discussed. Under O2/CO2 atmosphere, the main active sequence for NO reaction includes: NO→N→N2, and the main active path for NO reaction under air atmosphere is through N2→N→NO.