The Mg-8 mol% LaNi0.5 hydrogen storage materials were prepared by mechanically milling at cryogenic temperature (cryomilling). The phase structure and surface morphology of the materials were studied by XRD and SEM, the granularity distribution was determined by laser method, the hydrogen storage performance of the materials were studied by an equipment of PCT (pressure-composition-Temperature). The results showed that a small amount of the alloy phase appeared after ten hours. The average particle size of the materials is 10 μm. The materials had a flat platform of hydrogen absorption and desorption, the hydrogen storage capacity is 2.33 wt% at 573 K. Under 4.0 MPa hydrogen pressure, the materials can absorb more than 80% of their full hydrogen capacity in 3 min to 5 min at 523 K~653 K. The materials prepared by cryomilling had better activation performance, without activation, and fine platform performance, and the dynamic performance was improved