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The effects of a small amount of nonmetal elements (N, F and Cl) with high electronegativity interstitially doping on improving the hydrogen storage characteristics of Mg-based metal hydrides were systematically investigated by first-principle calculations in this paper. The interstitial positions which the doping elements easily occupied were firstly determined. The calculation results showed that these elements are most likely to hold the center position of octahedral sites with two Ni and four Mg atoms. Based on this, the crystal structures, thermal stability, dehydrogenation energy and electronic structures of all the crystals, including Mg2Ni, Mg2NiN0.5, Mg2NiF0.5, Mg2NiCl0.5 and their hydrides, were further investigated. The nonmetals with high electronegativity exhibit the favorable effects on the characteristics of Mg-based metal hydrides. Doping F significantly reduces the dehydrogenation energy of Mg2NiH4 by about 25%, because of the strong hybridization between F and H atoms. When doping Cl into Mg2Ni and Mg2NiH4, the formation enthalpies decrease respectively by 0.047 and 0.024 eV atom−1, due to the reduction of integral intensity of the bonding electron. Among the three elements, N has the best effects on improving both kinetics and thermodynamics. Doping N not only causes the formation enthalpies of Mg2Ni and its hydride to decrease by 0.215 and 0.141 eV atom−1 respectively, but also reduces the dehydrogenation energy of Mg2NiH4.