The four hydrogen storage systems including pure MgH2, MgH2-5 wt%NiCl2, MgH2-10 wt%NiCl2 and MgH2-10wt%NiCl2-10 wt%graphene were prepared by ball-milling in this work. Using experimental X-ray diffraction,scanning electron microscopy and differential scanning calorimetry testing methods in combination with firstprinciplecalculations, the dehydrogenation properties and mechanisms of NiCl2 single-doped and NiCl2-grapheneco-doped MgH2 composites were systematically investigated. Experimental results show that the NiCl2 singledopingis conductive to decreasing the size of MgH2 grains and particles. The co-doping of NiCl2 and graphene notonly reduces the size of MgH2 grains and particles, but also contributes to the uniformity of MgH2 particles. Ascompared with milled pure MgH2, the dehydrogenation peak temperatures are decreased by 24 °C and 47 °C forthe 10 wt%NiCl2 single-doped and 10 wt%NiCl2-10 wt%graphene co-doped MgH2 systems, respectively. It isdemonstrated that the co-doping of NiCl2 and graphene exhibits the synergistic effects of confinement and catalysison improving the dehydrogenation properties of MgH2. The first-principle calculations indicate that the codopingof NiCl2 and graphene leads to the distortion of MgH2 atomic-configuration and results in the charge transferbetween the dopants and MgH2, which induce the weakened structural stability and decreased dehydrogenationenthalpy of MgH2.