A novel 4LiAlH4 + Mg2NiH4 composite was synthesized. The synergistic hydrogen desorption effect has been discovered for the first time. Mg2NiH4 promotes decomposition of LiAlH4 in MM and thermal desorption process. In situ formed Al can destabilize Mg2NiH4 via a new reaction pathway. The composite shows ultrafast re-hydrogenation kinetics in initial three cycles.