Recent studies indicate that accurate description of the reaction mechanisms in fast pyrolysis of 1,3,5-trinitro-1,3,5-triazinane (RDX) is still of great challenge. In this work, the pyrolysis mechanism of RDX has been investigated by ab initio kinetic calculations and modeling simulations. The potential energy profiles of primary and secondary reactions in RDX decomposition were constructed with B2PLYPD3/CBS//B3LYP-D3/6–311+G(d,p), and the rate constants were obtained from the solution of RRKM/master-equations. The proposed reaction pathways and calculated rate constants were incorporated into a kinetic model. The simulated results illustrate that the decomposition of RDX mainly occurs through its NNO₂ bond fission to generate RDXR, whose subsequent decomposition mainly undergoes CH β-scission to form INT175 and the ring-opening reaction to form RDXRO. Interestingly, the less favored initial intermediate, INT175, is predicted to be a major product in the secondary decomposition of RDXR. The incorporation of important channels for key intermediates, such as CH β-scission of RDXR, remarkably affects the decomposing pathways of RDX. Meanwhile, the contribution of bimolecular reactions such as H/O abstraction reactions from RDX by NO₂/NO is simulated to be insignificant for the decomposition of RDX under the conditions studied.