In this study, a series of visible-light-driven BiOBrxI1-xsolid solution nanoplates photocatalysts are successfully prepared by an ultrasound-assisted hydrolysis method, which does not use organic reagents, with advantages of cost-effectiveness and non-toxicity. Under visible-light irradiation, all of the as-prepared BiOBrxI1-xnanoplates exhibit superior photocatalytic activities compared to those of pure BiOBr and BiOI for the degradation of methyl orange (MO). BiOBr0.3I0.7exhibits the highest photocatalytic activity, corresponding to the degradation of 92% MO in 40 min under visible-light irradiation. The structures and elemental composition of the as-prepared BiOBrxI1-xnanoplates samples are characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy. From the results obtained from X-ray photoelectron spectroscopy, UV–vis diffuse reflectance spectroscopy, and transient time-resolved luminescence decay, it is suggested that the enhanced photocatalytic activity of BiOBrxI1-xis possibly related to the narrowing of the band gap and high separation of the photo-generated electron–hole pairs. Electron paramagnetic resonance and mechanistic experiments indicated that [rad]O2−and h+are active radicals for photocatalytic degradation. In conclusion, an ultrasound-assisted hydrolysis method which is free of organic reagents is developed for synthesizing BiOBrxI1-xnanoplates photocatalysts with tunable bandgap and enhanced photocatalytic activity.