This study employed the reactive force field molecular dynamics to capture atomic-level heat and mass transfer and reaction processes of an aluminum nanoparticle (ANP) oxidizing in a high temperature and pressure oxygen atmosphere, revealing detailed mechanisms for oxidation of ANPs. Temporal variations of temperature, density, mean square displacement, atom consumption rate and heat release rate of ANP have been systematically examined. In addition, the effects of environment on ANP oxidation were also evaluated. The results show that ANP undergoes four stages of preheating, melting, fast Al core and moderate shell oxidations during the whole oxidation process. The Al core starts to melt from core-shell interface with outward diffusion of core Al atoms into the shell. Intense reaction occurs between shell O and core Al atoms around interface at the end of melting, leading to fast Al core oxidation. After complete oxidation of Al core, the oxide shell continues to react with ambient O atoms. Both the initial environmental temperature and the equivalent pressure significantly influence the preheating. Oppositely, the melting stage seems almost independent any of them. While the fast Al core oxidation presents more sensitivity to the ambient equivalent pressure.