MgH2 is a promising hydrogen storage material and its hydrogen desorption kinetics can be enhanced by introducing catalysts. To deeply understand synergetic effect of multiple phases on hydrogen desorption of MgH2, the formation mechanism of catalytic phases and their effect on hydrogen storage properties were studied in a MgH2 - 5 wt% TiF3 - 15 wt% NiCp2 composite. The experimental results demonstrate that the nano-sized Mg2NiH4, TiH2 and MgF2 phases can be formed in situ after sample preparation and activation. The in situ formed phases provide a synergetic effect on hydrogen absorption and desorption during subsequent cycling, leading to the significant enhancement of hydrogen desorption kinetics and cycle durability. This work provides guidance for the design and preparation of novel Mg-based materials with excellent hydrogen storage properties.