MgH2 is one of the ideal hydrogen storage materials because of its high hydrogen storage density, but the thermodynamic stability and slow kinetics of Mg/MgH2 system hinder its practical application. Transition metal catalyst doping has become an important strategy to improve the hydrogen storage kinetics of MgH2. In this work, FeNi3@CS was prepared by solvothermal method with carbon spheres (CS) as the support, and introduced into Mg/MgH2 system to prepare MgH2–FeNi3@CS composite. The initial dehydrogenation temperature of MgH2–FeNi3@CS composite is 565 K, which is 100 K lower than additive-free MgH2. At 423 K, MgH2–FeNi3@CS composite could absorb 4.5 wt% H2, while the hydrogen absorption capacity of MgH2 without additives is only 2.0 wt%. After 10 cycles, the hydrogenation and dehydrogenation capacity could be maintained at 5.6 wt% and 4.5 wt%, respectively. In addition, the activation energy of MgH2–FeNi3@CS composite decreased by 52 kJ/mol compared with MgH2. FeNi3 was in situ converted to Mg2NiH4 and Fe after hydrogenation-ball milling, which were considered as the catalytic active substances in Mg/MgH2 system. The synergistic catalysis of Mg2Ni/Mg2NiH4, Fe and carbon materials has a positive effect on improving the hydrogen storage performance of Mg/MgH2 system.