<p>While the Mg-Ni-H system represents a promising class of materials for hydrogen storage, it remains unclear which specific hydride phase offers the greatest potential for practical application. Here, the first-principles calculation method based on density-functional theory is employed to investigate parameters such as structural stability, hydrogen storage capacity, hydrogen dissociation energy, dielectric constant, and Young’s modulus of four hydrides (MgNiH, MgNiH_2, MgNiH₃, and Mg₂NiH₄). The results indicate that MgNiH₃ is the most stable with an enthalpy of formation of − 11.06&#xa0;eV. Although the hydrogen storage capacity of MgNiH₃ is 0.12% lower than that of Mg₂NiH₄, its hydrogen mass energy per unit volume is 20&#xa0;kg/m³ higher. Both MgNiH₃ and Mg₂NiH₄ exhibit a relatively high degree of polarization. However, the hydrogen dissociation energy of MgNiH₃ is higher than those of Mg₂NiH₄, which is conducive to the release of hydrogen. Meanwhile, MgNiH₃ also exhibits greater toughness, mainly due to the specific arrangement of the Ni 3d orbitals in different directions, resulting in different orbital arrangement sequences and strong anisotropy. Therefore, MgNiH₃ can serve as a more promising hydrogen storage material.</p> Graphical Abstract <p></p>