The electronic structure and dehydrogenation properties of NaMgH3 and metal(Li/K) substitution for parts of Na have been investigated using the density functional theory. Further calculations of reaction enthalpies along four possible dehydrogenation reaction pathways indicate that reaction pathway 5 is the most realistic ones. The Li doping in NaMgH3 decreases the value of reaction enthalpy and improves thermodynamic properties, but the K substitution has little impact on ones. The electronic densities of states and the mulliken population analysis show that Li doping is of benefit to releasing H because of decreasing the contribution of H to the valence band and weakening the interaction between Li and H. The different Li doping contents of reaction enthalpies are investigated indicate that with the increase of Li contents the structure present more excellent thermodynamic properties. But the Li substitution contents is less than 50.00%.