To illuminate the effects of eutectic structure on the de-/hydrogenation properties of Mg-based alloy, Mg-9Ni, Mg-12Cu and Mg-6Ni-3Cu (at. %) alloys with nearly the same primary Mg proportion are prepared. Mg2Ni and Mg2Cu respectively form a eutectic structure with Mg and tunable eutectic is obtained via Cu partial substituting Ni. Mg-6Ni-3Cu alloy shows excellent activation performance and rapid absorption rates without any sacrifice of desorption rates, leading to a maximum hydrogen capacity of 6.5 wt% H-2 under 300 degrees C 3 MPa. Different from the independent de-/hydriding behavior of Mg- > 12Cu alloy, Mg-6Ni-3Cu alloy shows perfectly wide pressure composition-isotherm plateau with minimal slope. The dehydrogenation models of experimental hydrides are determined. Owing to Cu substitution, the dehydrogenation activation energy reduces from 95.82 to 67.34 kJ mol(-1) H-2, the onset/peak desorption temperature also reduces significantly. It is the spatial connected eutectic and its increased boundaries that facilitate the activation and H diffusion process, which greatly compensate the negative impacts of reduced hydrogenation driving force caused by Mg2Cu. The rapid H diffusion coupled with the increased dehydrogenation driving force collectively facilitates its dehydriding process.