<p>This study examines the mix design and mechanisms of phosphogypsum slag composite (PSC) in solidifying and stabilizing compound heavy metal cations. The effectiveness of this composite to immobilize lead (Pb), cadmium (Cd), and mercury (Hg), as well as the micromorphology, distribution, and impacts of these metals within the matrix, were analyzed through toxic leaching tests, compressive strength tests, XRD, XPS, SEM-EDS, NMR, and MIP. Results show that a PSC comprising 50% GGBS, 45% PG, and 5% lime achieves optimal solidification/stabilization for Pb, Cd, and Hg, exhibiting significant mechanical strength and low leaching toxicity compared to controls. The PSC immobilizes heavy metals via chemical precipitation, forming insoluble compounds such as Pb₃(PO₄)₂, PbO, Pb₃Si₂O₇, Cd(OH)₂, and HgO. Additionally, PSC hydration products, particularly ettringite and calcium silicate hydrate, stabilize metal cations through physical encapsulation, adsorption, and lattice substitution with Ca. Competitive interactions among heavy metals significantly affect matrix strength development. The presence of multiple heavy metals was found to reduce ettringite formation, weaken silicate polymerization, and increase matrix porosity, thereby decreasing metal binding and raising leaching toxicity. This study provides a scientific basis and technical support for environmental remediation and resource recovery using this green material.</p>