By combining the ab initio molecular dynamics (AIMD) method and density functional theory (DFT), the thermal decomposition mechanism of s-tetrazine was studied. The dissociation channel was first simulated in a number of trajectories using the VASP package based on ab initio MD method, then further examined by Gaussian 98 at the B3LYP/6-311G(d,p) level to locate the minimum points and the transition structure. The high accuracy single point calculations were performed by B3LYP/6-311+G(2df,2p), G3MP2B3, G3B3, CCSD(T)/6-311G(d,p), CCSD(T)/ 6-311+G(2df,2p) and the rate constants were calculated by the microcanonical variational transition state theory (μVT). The studies suggest that s-tetrazine undergo concerted triple dissociation to form one N₂ and two HCN.