The Ni-25%X (X=Fe, Co, Cu, molar fraction) solid solutions were prepared and then doped into MgH₂ through high-energy ball milling. The initial dehydrogenation temperatures of MgH₂/Ni-25%X composites are all decreased by about 90 °C relative to the as-milled pristine MgH₂. The Ni-25%Co solid solution exhibits the most excellent catalytic effect, and the milled MgH₂/Ni-25%Co composite can release 5.19 wt.% hydrogen within 10 min at 300 °C, while the as-milled pristine MgH₂ can only release 1.78 wt.% hydrogen.More importantly, the dehydrogenated MgH₂/Ni-25%Co composite can absorb 5.39 wt.% hydrogen at 275 °C within 3 min. The superior hydrogen sorption kinetics of MgH₂/Ni-25%Co can be ascribed to the actual catalytic effect of in-situ formed Mg₂Ni(Co) compounds. First-principles calculations show that the hydrogen absorption/desorption energy barriers of Mg/MgH₂ systems decrease significantly after doping with transition metal atoms, which interprets well the improved hydrogen sorption properties of MgH₂ catalyzed by Ni-based solid solutions.