The invention discloses a preparation method of a photoelectric thin-film material based on BiOBr/CdS heterojunction. The preparation method comprises the following steps: adopting an aqueous solution of bismuth nitrate and potassium bromide as a reaction liquid, taking dry and clean FTO glass as a substrate, adopting a successive ionic-layer adsorption method to prepare 20-90 circulating BiOBr nano flaky array thin films; preparing a mixed precursor solution with 0.9-1.1M of ammonia water, 0.9-1.1mM of cadmium chloride and 4-6mM of thiourea, then vertically putting the prepared BiOBr nano flaky array thin films, and reacting at the temperature of 60-80 DEG C to obtain the photoelectric thin-film material based on the BiOBr/CdS heterojunction. The preparation method disclosed by the invention has the advantages that a series of problems of particle aggregation, microstructure damage, impurity introduction, high-temperature annealing, poor mechanical stability, high dropping possibility and the like caused in the traditional film-forming process are overcome; the service life of photon-generated carriers can be effectively prolonged and the transmission and separation efficiency can be improved, so that the photoelectric conversion efficiency is greatly improved.