Abstract: Solid-state hydrogen storage materials, as a recently popular class of hydrogen storage materials, have seen increased research focus on enhancing their hydrogen storage capacity and cycling stability. Investigating the structure-property relationship between the microscopic electronic structure characteristics and hydrogen storage performance of the lanthanum-nickel-based AB₃-type hydrogen storage alloy La_1−xMg_xNi₃ holds significant importance for developing highly efficient hydrogen storage materials. However, the hydrogen storage mechanism of La_1−xMg_xNi₃ alloys remains unclear and lacks theoretical support. Numerous researchers have investigated the hydrogen storage mechanism of lanthanum-nickel alloys using density functional theory (DFT), yielding preliminary conclusions. To explore the regulatory role of electronic behavior in the hydrogen storage mechanism of La_1−xMg_xNi₃, this study analyzes lattice constants, formation energies, charge densities, band structures, density of states, and partial wave density of states, aiming to further elucidate its hydrogen storage mechanism. Results indicate that LaNi₃ exhibits a formation energy of −0.33 eV per atom, confirming its strong bonding characteristics and high structural stability. The La₂MgNi₉ system shows a formation energy of −0.35 eV per atom, suggesting that magnesium substitution does not compromise structural stability while providing numerous interstitial sites that facilitate hydrogen diffusion and adsorption within the alloy. Analysis of charge density plots for the three alloys reveals that increasing Mg content expands the low-charge-density interstitial regions, providing more adsorption sites for hydrogen atoms. Simultaneously, the high-charge-density regions around Ni atoms are preserved, enhancing hydrogen storage capacity while maintaining stability in hydrogen-material interactions. The incorporation of Mg enhances the alloy's electron mobility and expands lattice voids, jointly optimizing hydrogen adsorption and diffusion properties. Among them, La₂MgNi₉ exhibits the most favorable comprehensive hydrogen storage potential. Density of states analysis indicates that increased Mg content transforms the alloy’s orbital hybridization from binary to ternary synergy, strengthening the interaction between electronic states near the Fermi level and hydrogen. Wavefunction density analysis reveals that Mg introduction transforms La’s d orbital hybridization from concentrated to dispersed, forming ternary synergy with Mg and Ni. This optimizes the electronic structure, significantly enhancing the alloy’s hydrogen adsorption and regulation capabilities. The combined computational results explain why both the La₈MgN₂₇ and La₂MgNi₉ alloys exhibit higher hydrogen storage capacities than 1.12% reported for LaNi₃.