Periodic first-principles calculations have been performed to study the effect of high pressure on the geometric, electronic, and absorption properties of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) under hydrostatic pressures of 0-50 GPa. Obvious irregular changes in lattice constants, unit-cell angles, bond lengths, bond angles, and band gaps showed that crystalline LLM-105 undergoes four structural transformations at 8, 17, 25, and 42 GPa, respectively. The intramolecular H-bonds were strong at pressures of 0-41 GPa but weakened in the range 42-50 GPa. The lengths of the intermolecular H-bonds (<1.47 Å) indicated that these H-bonds have covalent character and tend to induce the formation of a new twelve-membered ring. Analysis of the DOS showed that the interactions between electrons, especially the valence electrons, strengthen under the influence of pressure. The p states play a very important role in chemical reactions of LLM-105. The absorption spectrum of LLM-105 displayed more bands--as well as stronger bands--in the fundamental absorption region when the pressure was high rather than low. A new absorption peak due to O-H stretching appeared at 18.3 eV above 40 GPa, indicating that covalent O-H bonds and a new twelve-membered ring are present in LLM-105.