Nanosizing is efficient as the dual-tuning of thermodynamics and kinetics for Mg-based hydrogen storage materials. The in-situ synthesis of nanocomposites through hydrogen induced decomposition from long-period stacking ordered phase is proved effective to achieve active nano-sized catalysts with uniform dispersion. In this study, the Mg93Cu7-xYx (x = 0.67, 1.33, and 2) alloys with equalized Mg-Mg2Cu eutectic and 14H long-period stacking ordered phase of Mg92Cu13.5Y4.5 are prepared. Its solidification path is determined as alpha-Mg, 14H-Mg2Cu pair and Mg-Mg2Cu eutectic. The increased Y/Cu atomic ratio lowers the first-cycle hydrogenation rate of the alloys due to the increased 14H-Mg2Cu structure and reduced Mg-Mg2Cu eutectic interfaces. After the hydrogen-induced decomposition of the long-period stacking ordered phase, MgCu2 and YH3 nanoparticles are in-situ formed, and the following activation process significantly accelerates. The YH3 nanoparticles partly decompose to YH2 at 300 degrees C in vacuum and Mg-Mg2Cu-YHx nano-composites are in-situ formed. The nano-sized YH2 helps catalyze H-2 dissociation and the YHx/Mg interfaces stimulate H diffusion and the nucleation of MgH2. Therefore, the Mg93Cu5Y2 composite shows the fastest absorption rates. However, due to the positive effect of YHx/Mg interfaces on H diffusion and the negative effect of YH3 nanophases on the hydride decomposition, the minimum activation energy of 115.43 kJ mol(-1) is obtained for the desorption of the Mg93Cu5.67Y1.33 hydride. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.