Abstract: To address the potential risk of heavy metal groundwater pollution from multi-source coal-based solid waste backfill bodies in underground coal mining, this study selected four typical coal-based solid wastes—fly ash, coal gangue, furnace bottom ash, and gasification slag—and their corresponding cemented backfill materials as research objects. Using the acid solution buffering method (HJ/T299-2007) and Tessier five-step sequential extraction, combined with characterization techniques such as inductively coupled plasma mass spectrometry (ICP-MS) and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), we systematically investigated the enrichment characteristics, leaching levels, transformation of occurrence forms, and solidification mechanisms of heavy metals in dispersed, hydrated-cemented, and exogenous heavy metal (Cr³⁺/Zn²⁺)-modified backfill bodies. The results indicate: ① Enrichment and ecological risk: Cr content in solid wastes exceeded the Clark value of the Earth’s crust by 6～51 times. Ecological risk assessment revealed that gasification slag posed a “strong” risk (R₁ = 310) due to Cr and Hg enrichment, while coal gangue (R₁ = 282) and furnace bottom ash (R₁ = 244) showed “moderate” risk, and fly ash exhibited “slight” risk (R₁ = 141). ② Cementation and solidification effects: Hydration transformed heavy metals from soluble forms (exchangeable, carbonate-bound, and Fe/Mn oxide-bound) into stable, insoluble forms (residual and organic-bound). Solidification efficiency exceeded 50% for all heavy metals except Zn and Cd. At 2.0% exogenous Cr³⁺ addition, solidification efficiency exceeded 96% (leaching mass concentration: 346 μg/L), whereas Zn²⁺ showed less than 52% efficiency (leaching mass concentration: 38 775 μg/L) under identical conditions. ③Solidification mechanisms: Cr³⁺ underwent isomorphic substitution with Al³⁺ in C-A-S-H gel (confirmed by XRD peak shifts), forming stable chemical bonds for efficient solidification. Zn²⁺ primarily relied on physical adsorption within the porous C-S-H gel and clay minerals of coal gangue (SEM-EDS indicated similar distribution trends for Zn, Ca, and Na), resulting in lower efficiency. In conclusion, hydration cementation in coal-based solid wastes immobilizes heavy metals via four mechanisms—isomorphic substitution, chemical bonding, physical adsorption, and encapsulation—significantly reducing their mobility and environmental contamination potential.