Designing low-cost, highly efficient and robust bifunctional electrocatalysts for electrochemical overall water splitting remains a critical and urgent challenge for sustainable hydrogen production. Herein, a hierarchical Fe-modified CoMo sulfur-nitride heterostructure in-situ grown on three-dimensional porous nickel foam (Fe-CoMoNS/NF) is successfully fabricated via a facile two-step hydrothermal in-situ growth and simultaneous sulfurization-nitridation annealing strategy. In alkaline electrolyte, benefiting from the intense interfacial charge redistribution between heterogeneous components, abundant high-valent active sites induced by Fe doping and S,N co-modification, as well as the 3D open hierarchical structure for rapid mass and electron transfer, the as-prepared Fe-CoMoNS/NF only requires ultralow overpotentials of 62.9 mV for hydrogen evolution reaction (HER) and 188.2 mV for oxygen evolution reaction (OER) to achieve a current density of 10 mA cm⁻², respectively. When employed as a bifunctional electrocatalyst for alkaline overall water splitting, the assembled Fe-CoMoNS/NF||Fe-CoMoNS/NF electrolyzer delivers a current density of 10 mA cm⁻² at an ultralow cell voltage of 1.51 V, along with outstanding long-term stability for 100 h without obvious performance attenuation, outperforming most previously reported non-noble metal-based bifunctional electrocatalysts. This work provides a facile and effective strategy to develop high-activity, high-stability non-precious metal-based sulfur-nitrides as advanced bifunctional electrocatalysts for electrochemical overall water splitting.