The development of well-defined nanomaterials as non-noble metal electrocatalysts has broad application prospect for hydrogen generation technology. Recently, multi-metal electrocatalysts for hydrogen evolution reaction (HER) have attracted extensive attention due to their high catalytic performance arising from the synergistic effect of multi-metal interaction. However, most multi-metal catalysts suffer from the limited synergistic effect because of poor interfacial compatibility between different components. Here, a novel multi-metal catalyst (Ni/MoO@CoFeO) nanosheet with a crystalline/amorphous structure is demonstrated, which shows high HER activity. Ni/MoO@CoFeO exhibits an ultra-low overpotential of 18, 39, and 93 mV at 10 mA cm in alkaline water, alkaline seawater and natural seawater, respectively, which outperformances most of the state-of-the-art non-noble metal compounds. In addition, the catalyst shows exceptional stability under 500 mA cm in alkaline solution. In-situ Raman and other advanced structural characterization confirms the excellent catalytic activity is mainly contributed by: (1) the strong synergistic effect of multi-metal components provides multiple active sites in the catalytic process; (2) the crystalline/amorphous interface in Ni/MoO@CoFeO boosts the catalytically active sites and structure stability; (3) the crystalline phase enhances the intrinsic conductivity greatly; and (4) the amorphous phase provides abundant unsaturated sites for improved intrinsic catalytic activity. This work provides a feasible way to design electrocatalyst with high activity and stability for practical applications.