Nitramine compounds are typical high-energy-density materials (HEDMs) and are widely used as explosives because of their superior explosive performance over conventional energetic materials. In this work, the thermal properties of 1-nitropiperidine (NPIP), 1,4-dinitropiperazine (DNP), and 1,3,5-trinitro-1,3,5-triazinane (RDX) were investigated from quantum mechanics (QM) and reactive force field (ReaxFF) molecular dynamics simulations. We found that the bond dissociation energy of the N-NO2 bond, heat of formation, released energy, produced fragments, and oxygen balance are closely related to the incremental nitramine group. The nitramine group has a significant effect on the energetic performance of these nitramine compounds. In addition, the increase of the nitramine group will improve thermal decomposition activity, promote the generation of small molecules, and restrain the formation of carbon clusters. We hope that this work can shed new light on the design of energetic materials.