BiPO₄/Bi₄O₅Br₂ composites were synthesized via a straightforward solvothermal method and employed for the first time in photocatalytic N₂ fixation and degradation of dyes. The synthesized BiPO₄/Bi₄O₅Br₂ exhibited exceptional photocatalytic performance under simulated solar light. The N₂ fixation rate of BiPO₄/Bi₄O₅Br₂ was measured to be 370 μmol L⁻¹ g⁻¹ h⁻¹, and the degradation rates of MO and RhB within 90 min were 95% and 99.9%, respectively. Various techniques were employed to characterize the prepared BiPO₄/Bi₄O₅Br₂ photocatalysts. The results revealed negligible alterations in both the specific surface area and the range of light absorption. However, the charge separation efficiency experienced substantial improvement as a result of the electron drift upon contact between BiPO₄ and Bi₄O₅Br₂, leading to a change in band position and the formation of a type-II heterojunction structure. Hence, the improvement of charge separation was deemed the primary factor affecting the photocatalytic performance.