Among the factors which restrict the large-scale utilization of magnesium-based hydride as a hydrogen storage medium, the high operating temperature, slow kinetics, and air stability in particular are key obstacles. In this work, a novel method, namely hydriding combustion synthesis plus short-term mechanical milling followed by air exposure, was proposed to synthesize air stable and autocatalytic magnesium nickel hydride (Mg₂NiH₄), which shows excellent hydrogen absorption/desorption kinetics, capacity retention and oxidation resistance. The short-term-milled Mg₂NiH₄ can desorb 2.97 wt.% hydrogen within 500 s at 230 °C. Even after exposure under air atmosphere for 67 days, it can still desorb 2.88 wt.% hydrogen within 500 s at 230 °C. The experimental and theoretical results both indicated that the surface of as-milled Mg₂NiH₄ was easy to be oxidized under air atmosphere. However, the in-situ formed Ni during air exposure of Mg₂NiH₄ improved the hydrogen desorption kinetics, and the formed surface passivation layer maintained the hydrogen storage capacity and avoided further poisoning, which we called autocatalytic and self-protective effect. Such a novel dual effect modified the reaction activity and oxidation resistance of the air-exposed Mg₂NiH₄. Our findings provide useful insights into the design and preparation of air stable metal-based hydride for large-scale utilization and long-term storage.