Severe charge recombination in solar water-splitting devices significantly limits their performance. To address this issue, we design a frustum of a cone nanograting configuration by taking the hematite and Au-based thin-film photoanode as a model system, which greatly improves the photoelectrochemical water oxidation activity, affording an about 10-fold increase in the photocurrent density at 1.23 V versus reversible hydrogen electrode compared to the planar counterpart. The surface plasmon polaritons-induced electric field in hematite plays a dominant role in efficiency enhancement by facilitating charge separation, thus dramatically increasing the incident photon-to-current efficiency (IPCE) by over two orders of magnitude at the near-bandgap of hematite. And the relatively weak electric field caused by light scattering in the nanograting structure is responsible for the approximate maximum 20-fold increase in IPCE within a broadband wavelength range. Our scalable strategy can be generalized to other solar energy conversion systems.