Photocatalytic degradation is a new wastewater treatment technology, in which copper oxide (CuO) is a common photocatalyst. Herein, diatomaceous shale is used to load nanoscale CuO, which can, on the one hand, improve the photocatalytic performance, and on the other hand, reduce the cost of catalyst preparation. Furthermore, the morphology, crystal structures, and optical properties of the composite of CuO/diatomaceous shale are studied by scanning electron microscopy, X‐ray diffraction, UV–vis spectrophotometer, energy‐dispersive spectroscopy, and photoluminescence spectroscopy. Through the study of different experimental conditions, the reusability test demonstrates that the morphology and distribution status of CuO nanostructures are controllable. In addition, a group of composite catalysts prepared under the best technological conditions is selected to conduct the actual sewage degradation experiment, and the results show that the prepared composite of CuO/diatomaceous shale has a good degradation effect on sewage. These findings provide an insight into the design and synthesis of new CuO‐based composites to effectively improve their photocatalytic performance. [ABSTRACT FROM AUTHOR]