Fe-modified BiVO4 represents a promising anode material for the photoelectrochemical (PEC) oxygen evolution reaction in neural electrolytes, the bottleneck reaction in PEC water splitting. To reveal the catalytic role of Fe in this composite catalytic system, here we utilize combined theoretical and experimental techniques to identify the location and structure of FeOx phases and optimize the catalytic performance. By using the machine-learning interface search method, we screen out a coherent ε-FeOOH1.5(011)/BiVO4(001) interface from thousands of likely interface candidates. The interface has a low formation energy (0.74 J/m2), a narrow band structure (∼1.6 eV), and desirable catalytic activity (reaction barrier ∼ 0.64 eV) when the ε-FeOOH1.5 overlayer is two atomic layers thick. Guided by the theoretical findings, our orthogonal PEC experiments are performed to identify the optimal synthetic conditions. The best PEC activity reaches 5.4 mA/cm2 (1.23 V vs reversible hydrogen electrode) when using FeSO4 as the precursor with the chemical bath method at 40 °C for 4 h, which is ∼0.9 mV/ cm2 higher compared to the previous experiment. By analyzing transmission electron microscopy (TEM) pictures and performing TEM simulations, we confirm that the grass-like FeOx structures grown on BiVO4 are ε-FeOOH crystals as predicted by theory.