Design and synthesis of noble-metal-free bifunctional catalysts for efficient and robust electrochemical water splitting are of significant importance in developing clean and renewable energy sources for sustainable energy consumption. Herein, a simple three-step strategy is reported to construct cobalt-iron nitride/alloy nanosheets on nickel foam (CoFe-NA/NF) as a bifunctional catalyst for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The electrocatalyst with optimized composition (CoFe-NA2/NF) can achieve ultralow overpotentials of 73 mV and 250 mV for HER and OER, respectively, at a current density of 10 mA cm in 1 M KOH. Notably, the electrolyzer based on this electrocatalyst is able to boost the overall water splitting with a cell voltage of 1.564 V to deliver 10 mA cm for at least 50 h without obvious performance decay. Furthermore, our experiment and theoretical calculation demonstrate that the combination of cobalt-iron nitride and alloy can have low hydrogen adsorption energy and facilitate water dissociation during HER. In addition, the surface reconstruction introduces metal oxyhydroxides to optimize the OER process. Our work may pave a new pathway to design bifunctional catalysts for overall water splitting.