A 2LiBH 4 –MgH 2 –MoS 2 composite was prepared by solid-state ball milling, and the effects of MoS 2 as an additive on the hydrogen storage properties of 2LiBH 4 –MgH 2 system together with the corresponding mechanism were investigated. As shown in the TG–DSC and MS results, with the addition of 20 wt.% of MoS 2 , the onset dehydrogenation temperature is reduced to 206 °C, which is 113 °C lower than that of the pristine 2LiBH 4 –MgH 2 system. Meanwhile, the total dehydrogenation amount can be increased from 9.26 wt.% to 10.47 wt.%, and no gas impurities such as B 2 H 6 and H 2 S are released. Furthermore, MoS 2 improves the dehydrogenation kinetics, and lowers the activation energy (E a ) 34.49 kJ mol −1 of the dehydrogenation reaction between Mg and LiBH 4 to a value lower than that of the pristine 2LiBH 4 –MgH 2 sample. According to the XRD test, Li 2 S and MoB 2 are formed by the reaction between LiBH 4 and MoS 2 , which act as catalysts and are responsible for the improved hydrogen storage properties of the 2LiBH 4 –MgH 2 system.