Adding AlH3 nanoparticles (AHNPs) into energetic materials has become a widely used strategy to enhance the efficiency of heat release. However, the underlying mechanisms in the reaction between AHNPs and energetic materials are poorly understood. The evolution of AHNPs in 2,2 ',4,4 ',6,6 '-hexanitrodiphenyl ethylene (HNS) was simulated by reactive dynamic based on ReaxFF force field. As a whole, adding AHNPs unexceptionally increases heat release, and the particle size, passivation shell, and contents of AHNPs influence the morphological evolution and hydrogen release performance. AHNPs with smaller size can rapidly release hydrogen under the drastic micro-explosion. In addition, the aggregation mechanism of AHNPs is revealed at the earlier and later stages of the reaction. Al-O, Al-C, and Al-N bonds are generated during the reaction. A large amounts of H of AHNPs and C of HNS cause O to be desorbed from the Al-contained cluster. For the core-shell structure or/and larger size AHNPs, their oxidation process undergoes a transformation of H-2-H2O in the gas cavity. Carbon clusters formed during the reaction depend on the distribution of Al. The addition of AHNPs could promote the graphitization of carbon clusters. This work is expected to deepen insight into the reaction mechanism of AHNPs-containing energetic materials with negative oxygen balance.