The effect of Ce and Ni contents on the glass-forming ability (GFA) of Mg–Ce–Ni system in the Mg-rich corner of Mg–Ce–Ni system is revealed. Ce is more advantageous for the GFA of Mg-rich Mg–Ce–Ni system than Ni, and the lowest Ce content is ∼5 at.% to obtain the fully amorphous alloy. Amorphous alloy with the highest Mg content, Mg90Ce5Ni5, was obtained by melt-spinning. With the amorphous alloy as precursor, nanostructure multi-phases compositae was prepared by crystallizing it in hydrogenation process. The compositae with reversible hydrogen storage capacity of 5.3 wt.% shows much faster kinetics and lower MgH2 desorption activation energy than those of induction-melt Mg90Ce5Ni5 alloy. Both in situ formed nanosized Mg2Ni and CeH2.73 act as effective catalysts and significantly improve the hydrogen storage properties of MgH2. ► The effect of Ce and Ni contents on GFA of Mg-rich Mg–Ce–Ni system is revealed. ► GFR of Mg-rich Mg–Ce–Ni system is pictured for H-storage materials design. ► Mg90Ce5Ni5 owns the highest Mg content in amorphous Mg–Ce–Ni system. ► The mechanism of improved H-storage property of amorphous precursor is explained.