The initial thermal decomposition pathways as well as some important products generating mechanism of hexanitrohexaazaisowurtzitane(CL-20) /1, 3-dinitrobenzene(DNB)cocrystal at high temperatures(2000, 2500 K and 3000 K) were studied by reactive molecular dynamics simulations using ReaxFF force field. Results show that with the increasing of temperature during the thermal-decomposition process, the time to balance and potential energy decrease, while the quantity of products increases. The simulation results show that all the CL-20 molecules decompose faster than that of DNB, and as the temperature rises, the decomposition rate of DNB increases significantly. According to the product identification analysis, the main thermal decomposition products are NO₂, NO, N₂, H₂O, HNO₃, HON, HONO and CO₂ for cocrystal. The major initial decomposition mechanism is the breaking of N—NO₂ in the CL-20 and C—NO₂ in the DNB, which contributes to the formation of NO₂. And then, the number of NO₂ increases to the peak rapidly and decreased subsequently. After the NO₂→ONO rearrangement, it participates in other reactions and eventually occur N₂, NO, HONO, HON, H₂O and so on. In addition, the simulation results indicate that carbon-containing clusters formed in the later stage of decomposition at 2500 K and 3000 K, which is a common phenomenon during the detonation of rich carbon-containing explosives.