The electronic and optical properties of Ni-doped Bi₄O₅I₂ systems were investigated via first-principle calculations. For the pristine Bi₄O₅I₂, the band gap was calculated to be 2.17 eV, which was close to the experimental report. When the concentration of Ni increased, the band gap obviously decreased, which was ascribed to the introduction of Ni 3d doping states in the forbidden band gap. Based on the dielectric function analysis, the variations of the optical properties mainly took place in the visible light range, and the additional peak in the visible light range originated from the electron transition between the Ni 3d and Bi 6p states. Accordingly, the light absorption in the visible light range obviously increased, which was favorable for photocatalytic applications.