The isothermal section of Ce-Mg-Ni ternary system at 673 K was constructed through thermodynamic calculation coupled with experimental verification. Under guidance of the phase diagram in Mg-rich corner, Ce2.92Mg91.84Ni5.24 was designed to investigate its hydrogen storage properties. Moreover, the mechanism of alpha-to-beta (alpha: diluted phase and beta: hydride) phase transformation in the Ce2.92Mg91.84Ni5.24-H-2 system was elucidated by in situ synchrotron powder X-ray diffraction (SR-PXRD). The alloy showed a good activated ability and displayed two plateaus in the pressure composition temperature curves. The kinetic mechanisms of hydriding and dehydriding reactions were analyzed by Chou model. The rate-controlling steps were the diffusion for the initial stage in hydriding reaction and the surface penetration for dehydriding reaction. SR-PXRD patterns indicated that the activated Ce2.92Mg91.84Ni5.24 gradually transformed into CeH2.51, (Mg) (with solid solution of H atoms) and Mg2NiH0.3, finally transformed into CeH2.51, MgH2 and Mg2NiH4 during the first hydriding process. Copyright (c) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.