The explosive hazard of hydrogen peroxide (H2O2) and organics mixtures had drawn much attention, but the mechanism is still unclear. In this work, the atomic insights into the thermal runaway process of H2O2 and 1,3,5-trimethylbenzene (TMB) mixture was conducted using a new approach of combining reactive molecular dynamics (ReaxFF MD) and density function theory (DFT). The detailed reaction pathways were obtained through ReaxFF MD. The kinetic and thermal properties of main reaction steps were examined by DFT. This work divided the thermal runaway process into two stages. In stage I, H2O2 molecules were decomposed first to generate center dot OOH and center dot OH free radicals. The center dot OH radicals induced the initial oxidation of TMB molecular through H-abstraction and center dot OH-combine reaction steps with the highest thermal energy of 921.76 kJ/mol released, evoking the opening and cracking of benzene ring. In stage II, once the generated small molecules were further oxidized, the reactions showed a runaway for the massive thermal energy released, which explains the mechanism of larger potential risk of H2O2-organics mixture. Notably, center dot OH is the most crucial free radical carrier for the whole reaction process, the explosion hazard will be inhibited or weakened if the concentration of center dot OH radical is controlled. It is expected that this work will help researchers and industrial practitioners to better understand the intrinsic thermal hazard of H2O2-organics, and provide valuable guidance for the further development of efficient explosion suppression methods. (C) 2021 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.