TG-FTIR-MS techniques and ReaxFF molecular dynamics (ReaxFF MD) simulations are used to identify the main thermal decomposition intermediate and final products of 3-nitro-1,2,4-triazol-5-one (NTO). The study results show that the main intermediate products are C2HO3N4, C2H3O3N4, C2N, NO2 and the main final gas products are H2O, CO2, N2, NH3, H2, CH4, among which C2HO3N4 is the first generated intermediate product with the largest abundance and N2 is the first generated final product with the largest abundance. In addition, the main chemical reactions during NTO thermal decomposition are obtained by ReaxFF MD simulations. Based on the generation time of the intermediate products, final products and the main chemical reactions, three main decomposition paths of NTO are obtained, i.e. NTO → N2 + C2H2O3N2 → OH + C2HO2N2 → H + C2O2N2 → NO2 + C2N → …, NTO → H + C2HO3N4 → N2 + C2HO3N2 → OH + C2O2N2 → NO2 + C2N → … and NTO + H → C2H3O3N4 → H2O + C2HO2N4 → N2 + C2HO2N2 → NO2 + C2HN → H + C2N → … According to the experimental and simulation results, thermal decomposition mechanisms of NTO are obtained.