Nanoconfinement is an effective strategy for obtaining Mg-based hydrogen storage materials with fast reaction kinetics and decreased operating temperatures. However, the design of high loading nanoconfined Mg with an efficient catalyst remains a great challenge. Herein, we confined V-decorated Mg nanoparticles in 1 nm carbon shells through a reactive gas evaporation method. Due to the ultrathin carbon shells, the loading of the Mg–V@C nanocomposite reached over 94%. By adjusting the evaporation rate of Mg and V, the content of V in the nanocomposite could be accurately controlled from 2 to 25 wt%. Among the samples with different V contents, the Mg₉₂V₈@C nanocomposite with an average particle size of 32 nm had the best hydrogen storage properties. It showed a high hydrogen storage capacity of 6.6 wt% and could realize reversible hydrogenation/dehydrogenation cycles with over 5.2 wt% capacity at 473/573 K. The apparent activation energies for hydrogenation and dehydrogenation were reduced to 41 and 67 kJ mol⁻¹, respectively. The improved hydrogen storage properties are attributed to the nanoconfinement effect of the carbon shell and the catalytic effects of VH₂/V₂H nanoparticles as hydrogen pumps at different temperatures during hydrogenation and dehydrogenation.