Abstract: Semiconductors MoO₃, BiOI and g-C₃N₄ were compounded by a simple solvent thermal method, and finally, the ternary composite of double Z-scheme heterojunction BiOI(x)/MoO₃/g-C₃N₄ (x=6.25%, 12.50%, 18.75%, 25.00%, x: mass percentage of BiOI) was constructed. From the HRTEM results, it can be seen that two types of lattice fringes with spacings of 0.28 nm and 0.33 nm appear in the sample. Combined with the XRD characterization results, it could be seen that they respectively belong to BiOI (110) and MoO₃ (021) crystal planes, which thus indicats that the BiOI/MoO₃/g-C₃N₄ composite was successfully compounded, since g-C₃N₄ is amorphous material. The results of UV-Vis DRS show the decreased band gap and greater optical response range of the composite samples. The PL and photoelectrochemical test characterizations illustrate that the presence of the heterojunction effectively delays the recombination of electrons and holes. The dye methyl orange (MO) was degraded under simulated sunlight conditions, and its photocatalytic activity was studied. BiOI(18.75)/MoO₃/g-C₃N₄ had the best photocatalytic performance and optical stability, and the degradation rate of 30 mg/L MO was 94% for 120 min, which is 3.6 times that of pure g-C₃N₄. The ESR characterization indicates that the main active material components for the photocatalytic degradation of BiOI/MoO₃/g-C₃N₄ were ·OH and ·O₂^-. By calculating the positions of the valence and conduction bands of BiOI, MoO₃ and g-C₃N₄, it is shown that the three substances are band staggered structures, suggesting that the ternary complex forms a double Z-scheme heterojunction. BiOI/MoO₃/g-C₃N₄ may serve as an effective visible light-responsive catalyst for the degradation of organic dye pollutants, with promising applications.