<p>The long-term open-air storage of copper smelting slag (CSS) poses significant environmental hazards due to the release of heavy metals (HMs). However, the dynamic mechanisms governing the rainfall-driven migration and transformation of HMs in CSS remain insufficiently understood. This study investigated a CSS site in Sichuan, China, utilizing static leaching experiments to evaluate HMs contamination levels and column leaching experiments to simulate HMs migration and transformation dynamics under rainfall conditions. Multiple characterization techniques were integrated to elucidate the underlying mechanisms. Results indicated elevated HM concentrations in the slag, with Zn toxicity leaching exceeding hazardous waste identification thresholds. Column experiments revealed that rainfall, particularly acid rain events, significantly accelerates HM release and transformation. Under prolonged rainfall, HM release exhibited a biphasic pattern: an initial rapid phase followed by a stable phase, attributed to the rapid dissolution of ion-exchangeable fractions and the slow weathering of silicate minerals. Linear fitting of cumulative release data calculated annual HM fluxes of 4.32 × 10², 22.669 × 10⁴, and 9.9 × 10²&#xa0;g·ha⁻¹ for Pb, Zn, and Cu, respectively, with leachate samples frequently surpassing regulatory HM limits. Transformational behaviors were dominated by the oxidation of sulfidic fractions, which enhanced HM mobility and environmental risks. For example, the Risk Assessment Code (RAC) for Cu increased from 2.30% to &gt; 11% post-column experiments, elevating its risk classification to "moderate." This study dynamically investigates the release and transformation behaviors of HMs in CSS under rainfall conditions. The findings underscore the necessity for slag site managers to implement effective water-oxygen isolation measures or leachate collection systems to mitigate HM exposure and release.</p> Graphical Abstract <p></p>