We carried out reactive molecular dynamics. simulations by ReaxFF to study the initial events of an insensitive high explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) against various thermal stimuli including constant-temperature heating, programmed heating, and adiabatic heating to simulate TATB suffering from accidental heating in reality. Cluster evolution at the early stage of the thermal decomposition of condensed TATE was the main focus as cluster formation primarily occurs when TATB is heated. The results. show that cluster formation is the balance of the Competition of intermolecular collision and molecular decomposition of TATB, that is, an appropriate temperature and certain duration are required for cluster formation and preservation. The temperature in the range of 2000-3000 K was found to be optimum for fast formation and a period of preservation. Besides, the intra- and intermolecular H transfers are always favorable, whereas the C-NO2 partition was favorable at high temperature. The simulation results are helpful to deepen the insight into the thermal properties of condensed TATB.