A series of non-stoichiometric BiO_xI_3−2x photocatalysts were successfully prepared by a solvothermal method combined with thermal decomposition. Among them, BiO_1.18I_0.64 exhibited the highest photocatalytic activity. The degradation rate of aniline (50 mg L⁻¹) over the photocatalyst (1 g L⁻¹) was about 92.5% after 3 h of white LED lamp irradiation. Density functional theory calculations demonstrated that a much faster velocity of relative movement between electron and hole, as well as an apparently enhanced internal electric field in BiO_1.18I_0.64 greatly facilitated separation of photogenerated electron–hole pairs, which significantly improved its photocatalytic activity. In addition, I-vacancies in BiO_1.18I_0.64 could play an important role in capturing holes and thereby inhibiting the recombination of photoinduced electron–hole pairs. It further accounted for the obviously enhanced photocatalytic activity of BiO_1.18I_0.64.