The dehydrogenating properties of MgH2 systems were improved by the addition of metal fluorides as the catalyst; the energy of substitution for the Mg atom, the Mg vacancy formation energy, the energy to remove H atoms and electronic structure change of MgH2 systems were calculated by Dmol4.1 program based on the density functional theory. The mechanism of improvement of dehydrogenating properties of MgH2 systems by the addition of metal fluorides was discussed. The results show that the Mg vacancy formation energy is significantly higher than that of Fe, Ti, Zr, V, Ni, Nb, Cr or Cu substitution, which means that the substitution of these elements at the Mg site is more favorable for dehydrogenation of MgH2 system than the Mg vacancy formation at lower temperatures. NiF2, NbF5 and ZrF4 all benefit for the dehydrogenation of MgH2 systems, and the calculation results of improvement of the dehydrogenating properties of MgH2 systems are in agreement with experimental results. The Ni, Nb and Zr atoms replace a Mg atom of MgH2 systems respectively, the corresponding reactions of NiF2+3MgH2=MgF2+Mg2NiH4, 2NbF5+5MgH2=5MgF2+2NbH+4H2 and ZrF4+2MgH2=2MgF2+ZrH2 are accelerated; therefore, MgH2 with the higher stability can be changed into the other hydrides, such as Mg2NiH4, NbH and ZrH2. Compared with the density of states (DOS) of MgH2 systems, it is found that when the Ni, Nb or Zr atom replaces Mg atom, the valence electron numbers below Fermi level in octahedral areas made of the supercell center atom and its first-nearest, second-nearest H atom increased in the order of Ni, Ti, Zr, meaning the corresponding structural stability are increased while the dehydrogenating properties are reduced, both of which are in the order of Ni, Ti and Zr. A good agreement between the calculation results of the improvement of the dehydrogenating properties of MgH2 systems by NiF2, NbF5 and ZrF4 as the catalyst and experimental results can also be well explained