Water electrolysis technology is considered to be one of the most promising means to produce hydrogen. Herein, aiming at the problems of high overpotential and slow kinetics in water splitting, N-doped porous carbon nanofibers-coupled CoNi₂S₄ nanoparticles are prepared as bifunctional electrocatalyst. In the strategy, NaCl is used as the template to prepare porous carbon nanofibers with a large surface area, and sulfur vacancies are created to modulate the electronic structure of CoNi₂S₄. Electron spin resonance confirms the formation of abundant sulfur vacancies, which largely reduce the bandgap of CoNi₂S₄ from 1.68 to 0.52 eV. The narrowed bandgap is conducive to the migration of valence electrons and decreases the charge transfer resistance for electrocatalytic reaction. Moreover, the uniform distribution of CoNi₂S₄ nanoparticles on carbon nanofibers can prevent the aggregation and facilitate the exposure of electrochemical active sites. Therefore, the composite catalyst exhibits low overpotentials of 340 mV@100 mA·cm^–2 for oxygen evolution reaction and 380 mV@100 mA·cm^–2 for hydrogen evolution reaction. The assembled electrolyzer requires 1.64 V to achieve 10 mA·cm^–2 for overall water-splitting with good long-term stability. The excellent performance results from the synergistic effect of porous structures, sulfur deficiency, nitrogen doping, and the well-dispersed active component.