Magnesium and aluminum are abundant metals in the Earth’s crust and widely utilized in industrial engineering. Under high pressure, these elements can form elemental compounds into single substances, resulting in a variety of crystal structures and electronic properties. In this study, the possible structures of magnesium-aluminum alloys are systematically investigated in a pressure range of 0–500 GPa by using the first-principles structure search method, with energy and electronic structure calculations conducted using the VASP package. Bader charge analysis elucidates atomic and interstitial quasi-atom (ISQ) valence states, while lattice dynamics are analyzed using the PHONOPY package via the small-displacement supercell approach. Eight stable phases(MgAl₃-Pm\bar 3 m, MgAl₃-P6₃/mmc, MgAl-P4/mmm, MgAl-Pmmb, MgAl-Fd\bar 3 m, Mg₂Al-P\bar 3 m1, Mg₃Al-P6₃/mmc, Mg₃Al-Fm\bar 3 m) and two metastable phases (Mg₄Al-I4/m, Mg₅Al-P\bar 3 m1) are identified. The critical pressures and stable intervals for phase transitions are precisely determined. Notably, MgAl-Fd\bar 3 m, Mg₂Al-P\bar 3 m1, Mg₄Al-I4/m and Mg₅Al-P\bar 3 m1 represent newly predicted structures. Analysis of electronic localization characteristics reveals that six stable structures (MgAl₃-Pm\bar 3 m, MgAl₃-P6₃/mmc, MgAl-Pmmb, MgAl-Fd\bar 3 m, Mg₂Al-P\bar 3 m1 and Mg₃Al-P6₃/mmc) exhibit electronic properties of electrides. The ISQs primarily originate from charge transfer of Mg atoms. In the metastable phase Mg₄Al-I4/m, Al atoms are predicted to achieve an Al^5–valence state, filling the p shell. This finding demonstrates that by adjusting the Mg/Al ratio and pressure conditions, a transition from traditional electrides to high negative valence states can be realized, offering new insights into the development of novel high-pressure functional materials. Furthermore, all Mg-Al compounds display metallic behaviors, with their stability attributed to Al-p-d orbital hybridization, which significantly contributes to the Al-3p/3d orbitals near the Fermi level. Additionally, LA-TA splitting is observed in MgAl₃-Pm\bar 3 m, with a splitting value of 45.49 cm^–1, confirming the unique regulatory effect of ISQs on lattice vibrational properties. These results elucidate the rich structural and electronic properties of magnesium-aluminum alloys as electrodes, offering deeper insights into their behavior under high pressure and inspiring further exploration of structural and property changes in high-pressure alloys composed of light metal elements and p-electron metals.