The decomposition of nitramines explosives have been of great interest for a long time. However, theoretical investigations have concentrated mainly on unimolecular decomposition whereas bimolecular reactions have received only little attention. In this paper, the bimolecular reaction between NO2 with s-triazine (TAZ), which is an initial product during the decomposition process of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is investigated. The structures and potential energy surface (PES) are explored at B3LYP/6-31G(d,p) and B3P86/6-31G(d,p) levels, and the energies are refined using the CCSD(T)/cc-pVTZ methods. The mechanism of the reaction is analyzed. Quantum chemistry calculations reveal that the title reactions possess small barriers that can be similar to, or smaller than that of initial decomposition reactions of RDX, which suggests that bimolecular reactions are also of great importance, and should be further investigated. Moreover, the kinetics were investigated to verify the proposed mechanism of the reaction.