Renewable electrically driven water splitting technology is a crucial approach to obtaining clean hydrogen energy. However, the slow kinetics of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) significantly limit the overall efficiency. In this work, density functional theory (DFT) was first employed to design and verify the feasibility of introducing Cu to enhance conductivity, thereby improving catalytic performance. Subsequently, the NiFeCu electrocatalyst with a Cu-rich dendritic structure was synthesized via a magnetic field-assisted electrodeposition strategy. Numerical simulations by COMSOL and Fluent reveal the role and mechanism of the magnetic field, especially the ”magnetic diffusion effect” of improving the diffusion coefficient of Cu ions by reducing the radius of hydrated Cu2+ ions and the viscosity of the solution. Thermodynamic calculations show that the magnetic Gibbs free energy of the reaction process increases by 170 kJ mol−1 when a 0.5 T magnetic field is applied, proving that the magnetic field can accelerate the reaction. X-ray absorption fine structure (XAFS) analysis further confirmed that the Cu-rich structure donates electrons to Ni and Fe, optimizing the adsorption of intermediates. When the current density is 10 mA cm−2, the HER and OER overpotentials of NiFeCu-0.5 are 44 and 188 mV in 1 M KOH, and only 1.48 V ultra-low battery voltage is needed for overall water splitting. This research provides a new idea for designing high catalytic activity bifunctional catalysts and opens up a new direction for applying magnetic fields in material preparation and structural optimization.