Abstract: Stable non-noble metal bifunctional electrocatalysts are one of the challenges to the fluctuating overall water splitting driven by renewable energy. Herein, a novel self-supporting hierarchically porous Ni_xFe–S/NiFe₂O₄ heterostructure as bifunctional electrocatalyst was constructed based on porous Ni–Fe electrodeposition on three-dimensional (3D) carbon fiber cloth, in situ oxidation, and chemical sulfuration. Results showed that the Ni_xFe–S/NiFe₂O₄ heterostructure with a large specific surface area exhibits good bifunctional activity and stability for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) because of the abundance of active sites, synergistic effect of the heterostructure, superhydrophilic surface, and stable, self-supporting structure. The results further confirmed that the Ni_xFe–S phase in the heterostructure is transformed into metal oxides/hydroxides and Ni₃S₂ during OER. Compared with the commercial 20wt% Pt/C||IrO₂–Ta₂O₅ electrolyzer, the self-supporting Ni_1/5Fe–S/NiFe₂O₄||Ni_1/2Fe–S/NiFe₂O₄ electrolyzer exhibits better stability and lower cell voltage in the fluctuating current density range of 10–500 mA/cm². Particularly, the cell voltage of Ni_1/5Fe–S/NiFe₂O₄||Ni_1/2Fe–S/NiFe₂O₄ is only approximately 3.91 V at an industrial current density of 500 mA/cm², which is lower than that of the 20wt% Pt/C||IrO₂–Ta₂O₅ electrolyzer (i.e., approximately 4.79 V). This work provides a promising strategy to develop excellent bifunctional electrocatalysts for fluctuating overall water splitting.