By the first-principles calculations method based on the density functional theory, H2 adsorption and dissociation properties on clean, vacancy defective and Pd atom coadsorption Mg(0001) surfaces are investigated systematically. The calculation results show that the model of H2 adsorption on clean surface is weak physisorption, and there is a high energy barrier, i.e., 1.3774 eV, when H2 dissociates into two separate H atoms. Vacancy defect not only benefits enhancing of the physisorption interaction between H2 and Mg surface, but also decreasing of the energy barrier, i.e., 1.2221 eV, of H2 dissociation to some extent. For Pd atom coadsorption Mg(0001) surface, there is a strong chemisorption interaction between Pd atom and H2, and the energy barrier, i.e., 0.2860 eV, of H2 dissociation is reduced significantly. Further analysis of electronic structures shows that the catalytic activity for H2 adsorption and dissociation on three different surfaces is closely related to the bonding electrons number of s orbital of the topmost layer metal atoms which interact directly with H2 around Fermi level