A series of visible-light-responsive BiOBr_xI_1-x solid solutions were prepared by a rapid and efficient ultrasonication synthesis and applied in photodegradation of Rhodamine B in aqueous solution. The detailed characterisations showed that the lattice parameters and their band structures of the BiOBr_xI_1-x solid solutions significantly deviated from the well-established Vegard’s law for solid solution materials. The Mulliken electronegativity and valence band XPS analyses revealed that the substitution of Br by less electronegative iodine can simultaneously modulate the edges of conductance and valence band of the BiOBr, leading to nonlinear dependence of bandgap (Eg) on the halogen anion concentrations. Although the solid solution displayed superior RhB photodegration activity to BiOI, only Br-rich BiOBr_xI_1-x solid solutions (x>0.5) were more active than BiOBr and BiOI, with the optimal one is BiOBr_0.75I_0.25. The Br-dependence of bandstructure and photocatalytic activity for the BiOBr_xI_1-x solid solutions as well as their rate-limiting radical species were also clarified based on experimental and theoretical analyses.