This work reports a facile method for preparing the new photoactive In₂O₃ films as well as their implementation in photoelectrochemical (PEC) application. We firstly investigated the relationship between oxygen vacancies (V_O) and PEC performance and revealed a rule between them. We found that the optimized In₂O_3−n sample yielded a photocurrent density up to 3.83 mA/cm² in 1 M Na₂SO₄ solution under the solar illumination. It also gave efficiency as high as 75% over 400 nm in the incident-photon-to-current-conversion efficiency (IPCE) spectrum, which is the best value for an In₂O₃ photoanode reported. Moreover, the PEC performance of these films is enhanced as the V_O increased and then decreased with further increasing V_O. This two-side effect means V_O can favor the photoelectron activation, or act as recombination centers to prohibit the generation of photocurrent. Making highly photoactive In₂O₃ nanostructures in this work will open up new opportunities in various areas.