Isomerism is common in energetic compounds. Isomers may have differences in energy and safety performance. Investigating the mechanisms in these differences contributes to a deeper understanding of the structure-performance relationship of energetic compounds. The thermal decomposition mechanisms of three isomeric energetic compounds， 2，6-diamino-3，5-dinitro-1-oxide pyrimidine （LLM-105）， 3，5-diamino-4，6-dinitro-1-oxide diazine， and 1，4-dinitrofurazan ［3，4-b］ pyrazine （DNFP）， were studied using the self-consistent-charge density-functional tight-binding method （SCC-DFTB） under program heating and isothermal heating conditions. The results show that there is a strong hydrogen bond network in the LLM-105 crystal， enabling a molecular hydrogen transfer reaction accounting for 68.75% in the early stage of decomposition， which plays an important role in its high thermal stability； The skeleton structure of 3，5-diamino-4，6-dinitro-1-oxide diazine was prone to ring-opening through N─N bond cleavage under heating， resulting in lower thermal stability compared to LLM-105； The bond dissociation energy of DNFP for nitro group cleavage is 172.3 kJ·mol^-1， which is significantly lower than the other two isomers. Additionally， its fused-ring skeleton was also susceptible to ring-opening through C─C and N─O bond cleavage， resulting in the lowest thermal stability. In summary， the bond dissociation energy of the weakest bond in the molecule， the stability of the ring skeleton structure， and the hydrogen bond network of the crystal are important structural factors that determine the thermal stability of energetic compounds.