Rational design of non-noble metal electrocatalysts with high intrinsic activity for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is extremely impressive for sustainable electrocatalytic water splitting systems. However, it still remains a major challenge to engineer bifunctional performance. Here, we put forward a highly efficient water electrolyzer based on Ni₃S₂-based materials. The hierarchical structure of Ni₃S₂ can be well regulated for optimizing the HER catalytic activity. The best c-Ni₃S₂/NF electrode exhibits a much smaller overpotential of 220 mV to reach the current density of 100 mA cm⁻². Upon introducing Fe species onto the Ni₃S₂/NF electrode by a simple dipping/drying method, the intrinsic OER activity can be extremely improved. As a result, the Fe-c-Ni₃S₂/NF catalyst showed excellent catalytic activity for the OER, including an overpotential of 193 mV at 10 mA cm⁻², high specific current density and excellent stability. Post-characterization studies proved that the remaining S anions have an effective influence on improving the OER intrinsic activity. The assembled water electrolyzer also presented superior performance, such as a very low cell voltage of 1.50 V at 10 mA cm⁻² and excellent durability for 120 h in alkaline medium. This strategy provides a promising way to design highly active and low-cost materials for overall water electrolysis.