An innovative 3D flower-like BiOI/Bi₃O₄Br hierarchical heterostructures complex material was firstly constructed via in-situ growth of ultrathin BiOI on Bi₃O₄Br by a simple precipitation method. Under the simulated solar-light illumination for 90 min, the optimal 60% BiOI/Bi₃O₄Br composite displayed the highest photodegradation rate of 96.4% for RhB, 80.0% for TC and 77.2% for MO. The corresponding apparent rate constants were 0.0314, 0.0167 and 0.0110 min⁻¹, which were 8.26, 1.82 and 3.24-times than that of Bi₃O₄Br, 44.86, 11.13 and 2.62-times than that of BiOI. The improved photodegradation activity could be owing to the following reasons: 1) The 3D hierarchical heterostructures endowed BiOI/Bi₃O₄Br with tight interfacial contact, which can supply sufficient migration channels for photoinduced carriers; 2) The Z-scheme charge migrate pathway promoted the segregation of photocarriers while maintaining a robust redox ability; 3) The BiOI/Bi₃O₄Br heterostructures reinforced light absorption edges more than 600 nm and augmented specific surface areas over 60 m²·g⁻¹. In addition, the various photocatalytic conditions were studied to imitate the degradation process of RhB in the real aqueous system. The radical capture experiments and EPR test attested that h⁺ and O₂⁻ played an important part in the photodegradation of RhB.