This study calculates the electronic structure and optical properties of intrinsic BiOIO₃ and X-BiOIO₃ (X = As, Se, or Te) using PBE (Perdew–Burke–Ernzerhof) and MBJ (Modified Becke–Johnson) functionals based on density functional theory, with MBJ showing better correlation with experimental values. The X-BiOIO₃ systems exhibit relative stability under MBJ potential and show crystal lattice distortion compared to intrinsic BiOIO₃, creating localized potential differences that enhance polarization and adjust the bandgap. Doping reduces the bandwidth and increases energy level density, promoting electron transitions. Consequently, based on the computational results presented in this paper, it can be inferred that both BiOIO₃ and X-BiOIO₃ facilitate water hydrolysis and oxygen generation due to their favorable energy band positions. Notably, Se-BiOIO₃ exhibits the highest visible light absorption capacity, which may enhance photocatalytic efficiency by strengthening the built-in electric field and promoting charge carrier generation.