The invention belongs to the technical fields of new materials, new energy utilization and environmental pollution treatment, and relates to a preparation method of a high-activity MoS2/g-C3N4/Bi24O31Cl10 composite photocatalyst. The preparation method comprises the following steps: preparing Bi24O31Cl10 by taking bismuth nitrate, ammonium chloride and citric acid as raw materials through an improved solution combustion method; preparing g-C3N4 by taking melamine and acetic acid as raw materials through a one-step thermal polymerization method; preparing MoS2 by taking ammonium molybdate and thiourea as a raw material and dimethylformamide as a solvent through a hydrothermal method; and carrying out an ultrasonic mixing reaction on the Bi24O31Cl10 subjected to ultrasonic dispersion with the g-C3N4 and the MoS2 in a methanol solution, and then carrying out washing, centrifuging and drying to obtain the composite photocatalyst. The method is simple, is easy to implement, is low in cost and good in repeatability, and has a wide application prospect in the fields of water photodecomposition, photocatalytic oxidation of environment pollutants, and the like.