A preparing method of an oyster shell powder-copper oxide nanometer composite material is disclosed. The method includes 1) a step of pretreatment of oyster shell powder, namely a step of soaking waste oyster shell with diluted hydrochloric acid for a certain period of time, washing the oyster shell with an acid to remove organic and inorganic impurities on the surface, cleaning the oyster shell with distilled water, then putting the oyster shell into a drying oven, drying the oyster shell, crushing the oyster shell, and then sieving the crushed oyster shell with a sieve having a size of 80-300 meshes to obtain oyster shell powder; and 2) a step of preparing the nanometer composite material, namely a step of adding the obtained oyster shell powder into a copper nitrate solution having a concentration of 0.01-1.00 mol.L<-1>, soaking the oyster shell powder under stirring for 3-24 h, then performing filtration, drying an obtained product, and calcining the obtained product at 800-1100 DEG C for 2-10 h to obtain the nanometer composite material. The nanometer composite material is used for treatment of dye wastewater. The method is simple, stable, reliable and easy to operate, and thecomposite material has good organic dye adsorption performance when being used for treatment of dye wastewater.