The light absorption properties of semiconductor-based photocatalysts to a large extent determine the relevant catalytic performance. Traditional strategies in broadening the light absorption range are usually accompanied with unfavorable changes in redox ability and dynamics of photoinduced species that would confuse the comprehensive optimization. In this work, we propose a nontrivial excitonic transition regulation strategy for gaining sub-bandgap light absorption in low-dimensional semiconductor-based photocatalysts. Using bismuth oxybromide (BiOBr) as a model system, we highlight that the light absorption cut-off edge could be effectively extended up to 500 nm by introducing Bi vacancies. On the basis of theoretical simulations and spectroscopic analyses, we attributed the broadening of light absorption to the promotion of excitonic transition that is generally forbidden in pristine BiOBr system, associated with Bi-vacancy-induced excited-state symmetry breaking. In addition, Bi vacancy was demonstrated to implement negligible effects on other photoexcitation properties like excited-state energy-level profiles and kinetics. Benefiting from these features, the defective sample exhibits a notable advantage in gaining visible-light-driven photocatalytic reactions.