Although Mg metal features high volumetric energy density and electrochemical dendrite-free deposition, Mg-storage cathode materials with desirable capacity and long-term stability have reached a bottleneck due to large diffusion barrier of Mg2+. Herein, we report for the first time Ni-Fe bimetallic diselenides microflowers (Ni0.75Fe0.25Se2, NFS) as cathode materials for rechargeable magnesium batteries. The NFS exhibits a considerable reversible capacity of 190 mAh g−1 and excellent Mg-storage cycling stability (148 mAh g−1 even after 500 cycles). Compared with unary transitional-metal diselenides (NiSe2, NS), the NFS shows more redox active sites and higher Mg2+ diffusion dynamics, contributing to superior reversible capacity and long cycle life. Furthermore, the concept of sequential reaction based on the potential discrepancy for the NFS magnesiation/demagnesiation process at steady stage was put forward and evidenced by electrochemical measurement and structural characterization. This paper paves the way for constructing advanced high-performance cathode materials of rechargeable magnesium batteries.