The electronic structure and surface energy of Li2MgN2H2 were calculated by the first principle, and then the transition state of molecular H2 adsorption and dissociation on the lowest energy (100) surface was analyzed for re-hydrogenation of Li2MgN2H2. The results show that the Mg-Mg-Li site on the lowest energy (100) surface is the most stable for molecular H2 adsorption and dissociating, where the related energies for these two reactions are about –0.1898 eV and 0.84 eV (81 kJ/mol), respectively. This indicates that the activation energy for re-hydrogenation of Li2MgN2H2 is apparently too high, and it results in the slow reaction rate of hydrogenation.