The narrow band gap and the effective separation of photocatalytic reaction species are indispensable to use impurity-doped photocatalysts in photocatalytic environment remediation and solar fuels. Uniform porous BiOBr microspheres photocatalysts consisting of doped Ag⁺ and Er³⁺ ions were synthesized successfully via the microwave hydrothermal process. The results indicate that the degradation rate constants of RhB with Ag⁺-doped BiOBr and Er³⁺-doped BiOBr are 2.5 and 2.7 times stronger than that of pure BiOBr, respectively. Moreover, Ag⁺-doped BiOBr and Er³⁺-doped BiOBr can effectively capture photogenic electrons and inhibit the recombination of photogenic electron–hole pairs. Their ultraviolet–visible spectrum reveals that Ag⁺-doped BiOBr and Er³⁺-doped BiOBr with abundant energy levels exhibit a broader visible light response range and enhance visible light utilization. Based on density functional theory insights on the density of states, the band structures, and the charge density difference, the results reveal that Ag⁺-doped BiOBr and Er³⁺-doped BiOBr favor the separation of the photogenerated electron–hole pairs. The separated electrons and holes can be sufficiently activated by a redox reaction in both the conduction band and the valence band to complete the photodegradation process. In this work, the difference in the photocatalytic mechanism between Er³⁺-doped BiOBr and Ag⁺-doped BiOBr was systematically investigated. The results not only provide new insights into the doping properties of BiOBr but also promote its potential applications in semiconductor photocatalysts.