<p>Annual copper smelting slag output exceeds 40 million tons globally. Its massive accumulation causes resource waste and environmental problems, necessitating new utilization methods. These attempts are being used to treat wastewater and clean flue gas. This review summarizes the research in this field, highlights the existing problems, and provides guidance for further detailed studies and safe application. Copper slag serves as a catalyst for Fenton reaction in wastewater treatment, improves hydrogen peroxide, persulfate, and potassium permanganate oxidation abilities for organic pollutant degradation in wastewater, and acts as an adsorbent/precipitant for pollutant removal from wastewater. In flue gas cleaning, copper slag has been used as a catalyst for oxidation and removal of nitrogen oxide, sulfur dioxide, and mercury in flue gas. During treatment, harmful components in copper slag can be introduced into solution, and preprocessing can eliminate the risk. The use of copper smelting slag in water treatment and flue gas cleaning strategies expands its value. However, large amounts of heavy metals or organic compounds are adsorbed on copper slag during treatment processes, increasing the risk of secondary pollution. Hence, developing effective strategies for treating used copper slag is an increasingly important research topic. Herein, we propose an efficient solution for used copper slag, involving nonferrous metals and iron extraction, as well as the application of residual slag for making rock wool. This article provides a resource for expanding utilization channels and future research on copper slag.</p> Graphical Abstract <p></p>