Mg/C nanocomposites were compounded of C, Mg and Ni by reaction ball milling in hydrogen. The average size of nano-crystal in the composites is 16.8~33.6 nm. The absorption/desorption hydrogen performances show that the composites possess high reactivity and high hydrogen storage density. In the composite of 70Mg30C4Ni, 5.0 wt% hydrogen can be stored in 2 min at 160 °C and 2 MPa hydrogen pressure. The XRD analysis indicates that the synergistic action of Mg nano-crystal, nano carbon and the catalyst Ni is the main factor to the increase of hydrogen storage density and the improvement of dynamics performance. The chemical adsorption of carbon and pipeline of hydrogen in the Mg/C composite were studied by IR. Hydrogen is absorbed by C with priority and then analyzed to Mg, which is reversible.