Several different hydrogen storage systems including MgH2, MgH2-Graphene, MgH2-Ni and MgH2-Graphene-Ni were synthesized by high energy ball milling technique. The phase compositions, microstructures and dehydrogenation properties of the different systems were characterized by the testing methods such as X-ray diffraction, scanning electron microscope and differential scanning calorimeter. The effect and mechanism of dehydrogenation properties of MgH2 separate/multi-doped by Graphene and Ni were also studied systematically. It turned out that the initial dehydrogenation temperature of MgH2 doped by Graphene decreased by about 33℃ as a result of the effect of grain refinement and uniform size of MgH2 particles suffered from the structure confinement effect by Graphene in the ball-milling process. For the MgH2-Ni system, the initial dehydrogenation temperature of MgH2 significantly decreased by almost 136℃ due to the lattice deformation and the reduction of structural stability of MgH2 matrix where some Mg atoms were replaced by Ni atoms. However, the doping order played an important role in the Graphene and Ni multi-doped MgH2 system. When Graphene and Ni doped in the MgH2 system at the same time, the initial dehydrogenation temperature of MgH2 system did not reduce because the buffer function of MgH2 particle coated by Graphene made it difficult for the Ni atoms solid-soluted into the MgH2 matrix. While when Graphene doped in the MgH2 system after Ni atom, the initial dehydrogenation temperature further decreased by closely 175℃ with respect to pure MgH2 system due to both the effect of the realization of solid solution by Ni atom and the structure confinement effect by Graphene.