Seeking new methods to modulate the structure of metal–organic frameworks (MOFs) for diverse applications, particularly for water splitting, is intensively urgent but challenging. Herein, a simple hydrothermal method employing HCl as the modulator is developed to synthesize a series of NiFe-MOF-n/NF. The amount of HCl modulator not only changes the elemental composition and crystal structure but also modulates the electronic structure of NiFe-MOF-n/NF, thus improving intrinsic activity. Owing to the synergetic interactions between Ni and Fe atoms, free-standing feature, the optimized NiFe-MOF-2/NF yields excellent OER activity with overpotentials of 209 and 260 mV at 10 and 100 mA cm⁻², respectively, a small Tafel slope of 36.4 mV dec⁻¹ and excellent OER stability for 24 h at 100 mA cm⁻² in 1 M KOH. This demonstrates that NiFe-MOF-2/NF are in situ converted into metal oxide/oxyhydroxide after OER, thereby serving as the real active sites. This study offers a feasible way to fabricate low-cost, efficient MOF-based electrocatalysts.