The AgBr/BiOI/g-C3N4 ternary photocatalyst based on BiOI/g-C3N4 binary material was successfully prepared by a mild and simple method. The structure and morphology of the ternary materials were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), and Fourier transform infrared spectroscopy (FT-IR). The better optical properties of AgBr/BiOI/g-C3N4 were proved via ultraviolet-visible diffuse reflectance spectroscopy (DRS) and photoluminescence emission spectroscopy (PL), and the possible improved photocatalysis methods for degrading methyl orange (MO) were studied accordingly. MO (20 mg/L) was degraded by visible-light photocatalysis, and the optimum composite ratio of AgBr was 20%. At the optimum compounding ratio, the photocatalytic degradation rate of MO to the catalyst reached 93.41% at 120 min, which was attributed to the double Z-type heterojunction between AgBr, BiOI, and g-C3N4 and the strong electron capture effect of elemental Ag. The stability of AgBr/BiOI/g-C3N4 was examined by four cycles of experiments. Eventually, via trapping experiments, it was discovered that the primary active factor was ·O2-. Moreover, h+ also played a certain role. Based on this result, the possible photocatalytic mechanism was presumed, which was a double Z-type heterojunction was formed among AgBr, BiOI, and g-C3N4, and the strong electron capture effect of elemental Ag also played an important role.