<p>Stabilization/solidification (S/S) is a primary approach for the treatment of waste containing heavy metal ions. However, conventional silicate cement-based S/S systems often suffer from limited long-term stability and poor intrinsic compatibility. In contrast, basic magnesium sulfate cement (BMSC), characterized by rapid setting, early strength development, and high stability, demonstrates greater potential for heavy metal immobilization. Nevertheless, current studies lack a comprehensive assessment of the solidification performance and a clear understanding of its immobilization mechanisms. In this study, Cr³⁺ and Pb²⁺ were selected as target contaminants to systematically evaluate the physical and mechanical properties, leaching toxicity, hydration characteristics, microstructure, and phase composition of BMSC solidified bodies. The role of slag was also investigated to propose a more effective strategy for heavy metal immobilization. The main findings are as follows: BMSC exhibits multiple immobilization mechanisms, including lattice substitution, chemical bonding, and physical adsorption, which collectively enhance the efficiency and stability of heavy metal fixation. With increasing ion content, BMSC displays prolonged setting times, reduced hydration heat release rates, and delayed hydration progression. Concurrently, the principal diffraction peaks of the 5·1·7 phase decrease in intensity and shift position, while the phase morphology becomes more sparse. Furthermore, BMSC demonstrates superior immobilization for Cr³⁺ and exhibits notable water resistance. The incorporation of 30% slag further improves the durability of the solidified body. This work elucidates BMSC’s multi-pathway heavy metal immobilization mechanisms and demonstrates its feasibility for safe, efficient hazardous waste treatment, offering both theoretical insights and practical guidance.</p>