<p>Large-scale stockpiling of copper smelting slag (CSS) poses significant environmental risks, and its utilization in backfill mining offers a promising strategy for waste reduction and resource recovery. This study investigates an alkali-activated CSS-based filling material (CSSFM) prepared using cement clinker, gypsum, slaked lime, and NaOH as activators. Unlike previous studies that focused primarily on mechanical performance or incorporated fly ash as a secondary precursor, this work achieves a high slag incorporation rate of 69% while systematically examining mechanical behavior, heavy metal immobilization mechanisms, and leaching kinetics. The optimized mixture attained a 28-day uniaxial compressive strength of 32.43&#xa0;MPa, meeting the mechanical requirements for backfill applications. Leaching tests revealed that after 28&#xa0;days of curing, Cu²⁺, Zn²⁺, and As³⁺ concentrations decreased by 70.3%, 76.1%, and 64.7% respectively, with final values of 0.22, 0.16, and 0.30&#xa0;mg/L. All these values are well below the limits stipulated by the Chinese Hazardous Waste Identification Standard (GB 5085). Kinetic analysis showed that the pseudo-first-order model accurately described Cu²⁺ and Zn²⁺ leaching, with correlation coefficients of 0.989 and 0.953, while As³⁺ exhibited more complex behavior with a correlation coefficient of 0.848 and a longer half-life of 18.51&#xa0;days. SEM–EDS analysis further elucidated the immobilization mechanisms. Cu²⁺ was stabilized through Cu(OH)₂ precipitation and adsorption onto C–S–H gel. As³⁺ was immobilized via formation of insoluble calcium arsenate and isomorphous substitution of SO₄²⁻ by AsO₄³⁻ within the AFt structure. Zn²⁺ was primarily encapsulated within the C–S–H/AFt network. This study provides a mechanically reliable method for CSS reuse in backfill mining, with short-term leaching tests indicating compliance with regulatory standards for environmental safety.</p>