Abstract: Steel slag tailings, generated as a major solid waste during iron and steelmaking, are characterized by complex mineralogy and poor phase stability, which has resulted in persistently low resource utilization efficiency. To address the limitations of conventional leaching methods, including poor selectivity, low leaching efficiency, and restricted product utilization, this paper proposes an ammonium chloride-hydrochloric acid (NH₄Cl-HCl) synergistic leaching process, utilizing the high selectivity of ammonium salt leaching and the high leaching rate of hydrochloric acid leaching to achieve efficient extraction of calcium (Ca) and magnesium (Mg) while effectively suppressing iron (Fe) dissolution from steel slag tailings. The effects of hydrochloric acid concentration, ammonium chloride concentration, steel slag tailings particle size, leaching temperature, leaching time, and liquid-to-solid ratio on the leaching behaviors of Ca, Mg, and Fe were systematically investigated. Phase transformation and microstructural evolution during leaching were analyzed using X-ray diffraction (XRD) and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS). The results indicate that under the conditions of HCl concentration of 0.35 mol/L, NH₄Cl concentration of 2 mol/L, steel slag tailings with particle size less than 75 μm (200 mesh) accounting for approximately 45%, leaching temperature of 60 ℃, leaching time of 30 min, and liquid-to-solid ratio of 25.0 mL/g, the leaching efficiencies of Ca and Mg reached 61.20% and 20.75%, respectively, while the Fe leaching efficiency was limited to only 1.45%. Mechanistic analysis reveals that NH₄Cl alone can dissolve most of the active Ca and Mg bearing minerals in steel slag tailings, whereas the introduction of HCl further promotes the decomposition of calcium-magnesium silicate minerals. Meanwhile, the NH₄Cl-HCl synergistic system regulates the solution pH and induces hydrolysis and precipitation of iron species, resulting in preferential enrichment of iron in the leaching residue. SEM-EDS observations show that the leached slag surface exhibits pronounced porous corrosion features, with preferential dissolution of active calcium silicate minerals, while iron-bearing phases such as RO solid solutions and Ca₂(Fe, Al)₂O₅ remain largely undissolved. The research results provide both theoretical insight and practical guidance for the efficient separation and recovery of Ca, Mg, and Fe from steel slag tailings, and offer a promising technical route for the low-carbon and high-value utilization of steel slag tailings.