A ReaxFF reactive force field training set has been developed for the thermal decomposition of hydroxylammonium nitrate (HAN). The training set consists of geometries, partial atomic charges, and energy barriers for a number of reactions relevant to HAN thermal decomposition. Geometries and partial atomic charges were calculated in both the gas phase and in solution at the M06-2X/aug-cc-pVTZ level of theory. The SMD-GIL solvent model was used to approximate a high concentration of HAN in solution. Transition states for elementary reactions were found at the GIL/ωB97X-D/6-311++G** level of theory. An important autocatalytic pathway for the regeneration of HONO in HAN decomposition is discussed. The training set from this work can be used to train a ReaxFF force field capable of conducting reactive molecular dynamics simulations of HAN thermal decomposition.