The invention relates to a preparation method of a bismuth oxychloride composite magnetic photocatalyst and belongs to the field of an inorganic catalytic material. According to the preparation method, firstly, a precursor of the bismuth oxychloride/manganese zinc ferrite composite magnetic photocatalyst is prepared by adopting an impregnation method; then the product is obtained by filtration, washing, drying and calcinations. The method disclosed by the invention adopts a simple preparation process, is short in preparation period, is low in production cost, is low in calcinations temperature and is energy-saving. The bismuth oxychloride composite magnetic photocatalyst prepared by the preparation method has a narrow band gap, is high in photocatalysis activity, reaches a degradation rate of 99.6 percent for 10mg/L of rhodamine B solution in 30min in the simulated sunlight, and is easy to recycle; after the composite magnetic photocatalyst is used repeatedly for three times, the degradation rate for rhodamine B is still higher than 95 percent in 30min. The product prepared by the preparation method can be widely used in the field of photocatalysis degradation of organic pollutants.