Pseudopotential plane-wave method based on density functional theory was used to study influences of metal elements substitution on dehydrogenation of MgH₂. It shows that width of band gap and impurity energy levels are key factors. Impurity energy levels introduced by Fe, V, Nb, Ti are in the middle of energy gap, while those introduced by La are in the bottom of conduction band. Energy gap becomes narrower. The weakest Mg-H bond breaks easier, which is helpful to MgH₂ dehydrogenation. Covalent bonds formed between Nb, Fe, Ti, V, La and neighboring hydrogen lead to metal hydrides formation, which plays a catalytic role to dehydrogenation of MgH₂. Electrostatic forces between Mg₁ and surround hydrogens are no longer symmetrical duo to doping. H with weaker force to Mg releases easier, which improves dehydrogenation properties of MgH₂.