A plasmonic photocatalyst of AgBr/BiPO4/g-C3N4 was prepared. X-ray powder diffraction, Scanning electron microscope, Transmission electron microscopy, Fourier infrared spectroscopy, Ultraviolet Visible diffuse reflectance spectroscopy and photoluminescence emission spectra have been employed to determine the structure, morphology and optical property of the as-prepared AgBr/BiPO4/g-C3N4 composite and analysis the reasons for improving photocatalytic efficiency. The optimal doping ratio of AgBr was 10 wt% by degrading 20 mg/L of Reactive Blue 19 (RB19) under visible light (lambda > 420 nm), and 10 wt%AgBr/BiPO4/g-C3N4 degraded 20 mg/L of RB19 to 2.59% at 40 min, which is ascribed to synergistic effects at the interface of AgBr, BiPO4 and g-C3N4. The effect of catalyst dosage, initial concentration and initial pH of RB19 solution on photocatalytic efficiency was investigated. Four cycles of experiments were conducted. Finally, through the trapping experiment, we found that the main active factor for degrading RB19 in the photocatalytic process is O-center dot(2)-. The possible photocatalytic mechanism of AgBr/BiPO4/g-C3N4 was discussed in connection with the synergistic effect of Ag and active substances at the AgBr/BiPO4/g-C3N4 interface.