Using a density functional theory B3LYP method and on the 6-311+G(d, p) basis set level the hydrogenation reaction mechanism of Mg(NH2)2 and MgH2 was studied and the geometric configuration of the reactants, intermediates, transition states and products were optimized. To get more precise energy information, the single point energies of stationary points along the pathways were also calculated by QCISD/6-311+G(d, p) method, and the zero-point energy correction was performed for the total energy. Intermediates, transition states and the processes of the reaction were confirmed by the results of vibration analysis and the IRC calculation. Analyses indicate that the “a” of the first step hydrogen substitution reaction →the “f” of the second step hydrogen substitution reaction →the heterogeneous reactions of Mg3N2 was the main pathway, the activation energy of which was the lowest