It is reported that a three-dimensional cross-linked macromolecular structure with heterogeneous inorganic and organic compositions widely exists in coal particles. The macromolecules usually represent the rank transition of more than 75% of the carbon (C) content of coal particles. In order to know the coal combustion process better, it is important to specifically study the evolution of the coal macromolecule during combustion. In this paper, the structural evolution and the detailed oxidization reactions of a coal macromolecule during the process of combustion are numerically studied with the reactive force field (ReaxFF) molecular dynamics (MD) method, in which the carbon (C) and hydrogen (H) atoms are fully oxidized to CO2 and H2O, respectively. It is found that the coal macromolecule experiences three main stages sequentially: the cleavage, the ring opening, and the oxidation. The heteroatoms (O, N, and S) inside the coal macromolecule are found to play important roles throughout the whole combustion process. The detailed chemical reactions with their occurrence frequencies show that the chemical reactions with O2 mainly occur in C1–4 fragments, and the C1–2–H–O fragments widely exist in the system before they are finally oxidized to CO or CO2.