In order to understand the photophysical properties and explain the experimental observations of Eu-doped BiOX (X = F, Cl, Br, I), the crystal structure, electronic structure, and optical properties of pure BiOX and Eu-doped BiOX have been calculated using the DFT+U method. By Eu doping, the band gap of BiOI is slightly narrowed, while the band gaps of others (BiOF, BiOCl, and BiOBr) are slightly broadened. Importantly, there is an isolated impurity energy band in the middle of the band gap, which is formed by seven spin-up energy levels of Eu-4f states. Furthermore, Eu doping enhances the internal electric fields of BiOX and makes the variation of band gaps and band widths become more outstanding, especially for the band widths of X-ns, O-2s, and Bi-6s related bands. Taking into account the energy level matching and band edge position, Eu-doped BiOCl is favorable not only for the photoluminescence application but also for the photocatalysis application. The findings in the present work could well explain the experimental observations in the literature and are helpful for the development of novel optoelectronic applications of BiOX-based materials.