The thermal decomposition of solid nitromethane (NM) is studied by ReaxFF molecular dynamics simulations to obtain the time evolution of the mechanism of NM under high temperature and pressure. It is determined that the initial decomposition mechanism of NM is dependent on pressure effect. In the 0–3 GPa pressure regime, the initial reactions is the C–N bond dissociation and the unimolecular rearrangement connecting between NM and methyl nitrite isomers; in the 4–7 GPa, the initial pathways of NM are the intermolecular proton transfer and C–N, C–O bond rupture. In the secondary reactions step, several fragments, like H2O, NO, NO2, HONO, play a role of catalysis. The product decomposition of NM contains many different structures of carbon clusters, and the configuration of cluster is dependent on pressure.