The pyrolysis processes of a coal particle containing 19,638 atoms in different atmospheres are studied with a reactive force field molecular dynamics (ReaxFF MD) method. The detailed chemical reactions with the corresponding occurrence frequencies are obtained. The generation paths of the main products are disclosed, including CO, H2, H2O, and CH4. The nonuniform effect of temperature on the pyrolysis production is analyzed, among which the productions of CH4 and CO nonmonotonically vary with temperature, while the H2 production increases linearly with temperature. The kinds of atmospheres can significantly influence the coal pyrolysis. Hydrogen atmosphere can apparently improve the CH4 production, which results from the enhancement of the C–H bond generation.