The thermal decomposition of condensed CL-20 was investigated using reactive force field molecular dynamics (ReaxFF MD) simulations of a super cell containing 128 CL-20 molecules at 800-3000 K. The VARxMD code previously developed by our group is used for detailed reaction analysis. Various intermediates and comprehensive reaction pathways in the thermal decomposition of CL-20 were obtained. Nitrogen oxides are the major initial decomposition products, generated in a sequence of NO2, NO3, NO, and N2O. NO2 is the most abundant primary product and is gradually consumed in subsequent secondary reactions to form other nitrogen oxides. NO3 is the second most abundant intermediate in the early stages of CL-20 thermolysis. However, it is unstable and quickly decomposes at high temperatures, while other nitrogen oxides remain. At all temperatures, the unimolecular pathways of N-NO2 bond cleavage and ring-opening C-N bond scission dominate the initial decomposition of condensed CL-20. The cleavage of the N-NO2 bond is greatly enhanced at high temperatures, but scission of the C-N bond is not as favorable. A bimolecular pathway of oxygen-abstraction by NO2 to generate NO3 is observed in the initial decomposition steps of CL-20, which should be considered as one of the major pathways for CL-20 decomposition at low temperatures. After the initiation of CL-20 decomposition, fragments with different ring structures are formed from a series of bond-breaking reactions. Analysis of the ring structure evolution indicates that the pyrazine derivatives of fused tricycles and bicycles are early intermediates in the decomposition process, which further decompose to single ring pyrazine. Pyrazine is the most stable ring structure obtained in the simulations of CL-20 thermolysis, supporting the proposed existence of pyrazine in Py-GC/MS experiments. The single imidazole ring is unstable and decomposes quickly in the early stages of CL-20 thermolysis. Many C-4 and C-2 intermediates are observed after the initial fragmentation, but eventually convert into stable products. The distribution of the final products (N-2, H2O, CO2, and H-2) obtained in ReaxFF MD simulation of CL-20 thermolysis at 3000 K quantitatively agrees with the results of the CL-20 detonation experiment. The comprehensive understanding of CL-20 thermolysis obtained through this study suggests that ReaxFF MD simulation, combined with the reaction analysis capability of VARxMD, would be a promising method for obtaining deeper insight into the complex chemistry of energetic materials exposed to thermal stimuli.