Reactive molecular dynamics was used to investigate the atomic-level mechanism of formic acid-accelerated deterioration of meta-aramid (PMIA) fibers. The simulation results showed that formic acid promoted PMIA decomposition. The activation energy of a composite system (PF) consisting of formic acid and PMIA was 106.94 kJ/mol at 2000-3000 K, which is 11.95% lower than that of pure PMIA. The main small-molecule products of the PF system were H/C/O-containing molecules (H2O, CO, and CO2), hydrocarbon molecules (e.g., CH4, •C2H, C2H4, and C3H4), N-containing molecules (N2, NH3, and HCN), H2, and various free radicals. Formic acid can promote the production of small molecules such as CO, CO2, and H2O. The N-H bonds, C-N bonds and the amide C═O double bond of PMIA were vulnerable to CO, H ions, and free radicals produced by formic acid decomposition, and this decreased the PMIA stability. Temperature is an important factor in the thermal decomposition of PMIA and can accelerate reactions in the PF system. The initial reaction rate of PMIA at 3000 K was 8.1 times that at 2000 K, and the intermediate reaction rate was 6.2 times that at 2200 K; temperature also affects the types of pyrolysis products, for example, hydrocarbons are high-temperature products.