The improvement of the hydrogen storage mechanism in TM-doped MgH2 by structural high-pressure effects has been found using ab initio calculation. Phase transition, formation enthalpy and H-vacancy mechanism of alpha-, beta-, and gamma-MgH2 with 3.125% of Ni, Pd and Pd dopants are analyzed under the pressure conditions of 0, 5 and 10 GPa. It is found that the enthalpy of beta- and gamma-phases based on the alpha-phase decreases in TM-doped systems, especially for the heavier atomic size of dopants (Pt > Pd > Ni). As a result, the gamma-phase has become structural stable at ambient pressure. The occupation enthalpy of TM substitutions in beta and gamma phases is easier than that in the alpha phase, which indicates ability of mixing impurities. High pressure induces the occupation of H-vacancy in all compounds. The activation energy curves of MgH2 with Ni, Pd and Pt dopants are also analyzed, and the minimal barriers are significantly dominated in the gamma phase. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.