The Ag3PO4 and Ag3PO4 sonocatalysts with surface enrichment of Br− anions (Br–Ag3PO4) were synthesized using a facile precipitate process. Various analysis, including X-ray diffraction, transmission electron microscopy, scanning electron microscope, Brunauer–Emmett–Teller, photoluminescence (PL) spectra, and UV–Vis absorption spectra were carried out to investigate the property and structure. The effect of a variety of changing conditions on ultrasonic degradation of organic dye has been investigated. Both of them exhibited obvious activity for sonocatalytic degradation of organic dyes. However, Br–Ag3PO4 sonocatalysts showed much higher activity than that of Ag3PO4. By XRD analysis, the Br− anions of the surface of Ag3PO4 has been transformed into AgBr in the preparation process. The high activity could be attributed to AgBr for modifying Ag3PO4. By detecting hydroxyl radicals (·OH) in the reactive solution, we found that AgBr on the surface of Ag3PO4 can help to produce a lot of additional ·OH radicals. These ·OH radicals may be significant reason to improve Ag3PO4 activity, which further proved the ·OH radical mechanism of Ag3PO4 degradation of dyes under ultrasonic radiation. In addition, kinetic analysis and sonocatalytic mechanism were discussed in detail.