A thorough investigation of the initial decomposition pathways of triazoles and their nitro-substituted derivatives has been conducted using the MP2 method for optimization and DLPNO-CCSD(T) method for energy. Different Mow initial thermolysis mechanisms are proposed for 1,2,4-triazole and 1,2,3-triazole, the two kinds of triazoles. The higher energy barrier of the primary decomposition path of 1,2,4-triazole (H-transfer path, similar to 52 kcal/mol) compared with that of 1,2,3-triazole (ring-open path, similar to 45 kcal/mol) shows that 1,2,4-triazole is more stable, consistent with experimental observations. For nitro-substituted triazoles, more dissociation channels associated with the nitro group have been obtained and found to be competitive with the primary decomposition paths of the triazole skeleton in some cases. Besides, the effect of the nitro group on the decomposition pattern of the triazole skeleton has been explored, and it has been found that the electron-withdrawing nitro group has an opposite effect on the primary dissociation channels of 1,2,4-triazole derivatives and 1,2,3-triazole derivatives.