To improve the hydrogen storage property of La-Y-Mg-Ni alloy, the La_1.7Y_0.3Mg₁₆Ni alloys with an amorphous and nanocrystalline structure were prepared by ball milling the as-cast alloy for 5-30 h. The effects of microstructure on hydrogen storage property and its mechanism were analyzed. The results show that with prolonging milling time, the crystallinity, grain size and particle size decrease and the amorphous phases increase. The dual regulation of the nanocrystalline and amorphous phases results in the hydrogen storage kinetic property increasing at first and declining later. The 15 h-milled alloy gets the best hydrogenation and dehydrogenation kinetic property, which can absorb 3.10 wt.% hydrogen gas in 10 min at 373 K and has the lowest dehydrogenation activation energy of 71.2 kJ/mol. The thermodynamic property of the alloys milled for different time shows a little change, and the 15 h-milled alloy has the lowest dehydrogenation enthalpy change of 72.9 kJ/mol.