A new YMg4Cu intermetallic with orthorhombic TbMg4Cu type structure was prepared by induction melting and subsequent heating treatment. Hydrogenation of YMg4Cu resulted in phase segregation into the YH3-MgH2-MgCu2 nanocomposite. The phase components, microstructure and hydrogen ab/desorption kinetics of Y-Mg-Cu-H nanocomposite were carefully investigated. The in-situ formed Y-MgCu-H nanocomposite can absorbed 3.0 wt% hydrogen at 523 K, absorbed 2.6 wt% hydrogen at 373 K under 4 MPa H-2 and desorbed 3.0 wt% hydrogen under an initial hydrogen pressure of 0.001 bar at 550 K. The PCT curves show two plateaus corresponding to the formation of Mg and Mg2Cu, and the hydrogen desorption activation energy is -72.3 and -76.8 kj/mol H-2, respectively. Furthermore, the in-situ formed Mg/MgH2, Mg2Cu/MgH2+MgCu2 matrix and YH2/YH3 nanocatalysts is crucial to maintain the high effective capacity of this nanocomposites.