A robust water oxidation system using BiVO₄ as light-harvesting material and iron-containing polyoxometalate Fe11 as cocatalyst is constructed, in which an apparent quantum efficiency of 25.3% and a turnover frequency (TOF_initial) of 173 h⁻¹ are obtained. Most importantly, the iron-containing polyoxometalate remains stable and intact during the reaction process through the pH adjustment strategy and plays a true molecular cocatalyst role in the semiconductor photocatalytic water oxidation system.