<p>This study employs a multi-scale collaborative research framework to elucidate the thermal decomposition mechanism of nitroguanidine and ensure its safe application, integrating differential scanning calorimetry (DSC), ReaxFF (ReaxFF Molecular Dynamics, ReaxFF-MD) simulations, and in situ Fourier transform infrared spectroscopy (FTIR). The combination of DSC experiments and molecular dynamics simulations revealed a three-stage thermal decomposition pathway for nitroguanidine. The first stage involves the preferential cleavage of weak intramolecular N–N bonds, resulting in the formation of guanidyl and nitro radicals. In the second stage, the nitro radical abstracts hydrogen to produce HNO₂, which quickly decomposes into NO, NO₂, and H₂O. The guanidyl radical undergoes rearrangement of the C–N bond, leading to the production of N₂, cyanamide, and hydrogen radicals, which establishes a radical chain reaction cycle. The third stage involves the formation of gaseous products through several pathways, including the decomposition of nitric acid, the conversion of isocyanic acid, and the cleavage of amino intermediates. Solid residues mainly form under anoxic or incomplete decomposition conditions, primarily consisting of nitrogen-containing organic derivatives with carbon cores and guanidine salt compounds. These residues originate from the polymerization products of insufficiently oxidized cyanamide and cyano radicals, or from neutralization reactions between undecomposed guanidine groups and acidic substances. This study provides both experimental and theoretical evidence to enhance the understanding of the multi-step pyrolysis mechanism and product evolution patterns of nitroguanidine. It offers essential theoretical support for the quantitative assessment of thermal stability and performance improvement.</p>