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The combination of Al nanoparticles (ANPs) as fuel and H₂O₂ as oxidizer is a potential green space propellant. In this research, reactive force field molecular dynamics (ReaxFF-MD) simulations were used to study the influence of water addition on the combustion of Al/H₂O₂. The MD results showed that as the percentage of H₂O increased from 0 to 30%, the number of Al-O bonds on the ANPs decreased, the number of Al-H bonds increased, and the adiabatic flame temperature of the system decreased from 4612 K to 4380 K. Since the Al-O bond is more stable, as the simulation proceeds, the number of Al-O bonds will be significantly higher than that of Al-H and Al-OH bonds, and the Al oxides (Al[O]_x) will be transformed from low to high coordination. Subsequently, the combustion mechanism of the Al/H₂O₂/H₂O system was elaborated from an atomic perspective. Both H₂O₂ and H₂O were adsorbed and chemically activated on the surface of ANPs, resulting in molecular decomposition into free radicals, which were then captured by ANPs. H₂ molecules could be released from the ANPs, while O₂ could not be released through this pathway. Finally, it was found that the coverage of the oxide layer reduced the rate of H₂O₂ consumption and H₂ production significantly, simultaneously preventing the deformation of the Al clusters’ morphology.