A method for preparing a magnesium-based hydrogen storage material, includes: a Mg—Ce—Ni family amorphous alloy is prepared by a rapid cooling process; the amorphous alloy is pulverized, so as to obtain a amorphous powder; the amorphous alloy is activated, so as to obtain a MgH2—Mg2NiH4—CeH2.73 family nanocrystalline composite; the abovementioned composite is carried out a hydrogen absorption and desorption cycle, then the composite is placed in a pure Ar atmosphere for passivation, finally, the passivated composite is oxidized, so as to obtain a MgH2—Mg2NiH4—CeH2.73—CeO2 family nanocrystalline composite.