The band structure-controlled solid solution of BiOBrxI1-x was successfully synthesized by a simple solvothermal route. The prepared samples were characterized by X-ray diffraction, scanning electron microscopy, UV-vis diffuse reflectance spectroscopy, and nitrogen sorption/desorption. The resulting BiOBrxI1-x samples were phase-pure and of three-dimensional (3D) microspheres composed of nanoplates. The samples with different x values exhibited composition-dependent absorption properties in the visible light region and the bandgaps were estimated to be between 1.89 and 2.53 eV. Rhodamine B (RhB) photocatalytic degradation experiments showed that these samples possessed excellent and composition-dependent performance. The highest catalytic performance of the 3D BiOBr0.2I0.8 microspheres may derive from a synergetic effect, including higher surface area, porous structure, and enhancement of light absorbance. Moreover, on the basis of the analysis of the valence band and conduction band, a possible mechanism of photocatalytic activity of BiOBrxI1-x samples was also proposed.