Abstract: Using modified phosphogypsum (PG) as a road subgrade material represents a promising pathway for the resource utilization of industrial solid waste. However, pollutants such as fluoride contained in it pose potential environmental and health risks during long-term service. We modified PG chemically using a stabilizing agent containing reactive silicon- and aluminum-bearing components, promoting the formation of multiple stable mineral phases through hydration reactions. Then we conducted a comprehensive characterization of the microstructure and chemical composition after modification. The results showed that fibrous calcium silicate hydrate, calcium aluminosilicate hydrate and rod-like ettringite crystals were generated during the modification process, forming a dense spatial network structure that effectively immobilized fluoride and heavy metals. Subsequently, we performed tank leaching tests to investigate the fluoride leaching behavior of the modified PG. Combined with release mechanism analysis and human health risk assessment, the leaching characteristics, long-term release behavior and environmental safety of the modified material were systematically evaluated. Leaching results showed that fluoride concentrations exhibited a distinct "rise-stabilization-gradual decline" pattern over time, while cumulative release increased monotonically throughout the 64-day test, indicating a persistent and slow-release behavior. The release mechanisms shifted from an initial retardation phase to a diffusion-controlled release. The dual-constant equation best described the release dynamics, with predicted 15-year cumulative fluoride release amounts being 713.12 and 590.76 mg/kg for stockpiled and continuously produced PG samples, respectively, both substantially lower than the initial fluoride content (950 mg/kg). Furthermore, leachate concentrations of fluoride and heavy metals complied with the Class Ⅰ limits of Integrated Wastewater Discharge Standard (GB 8978-1996). The health risk assessment showed that the non-carcinogenic hazard quotients for both samples were below 1, indicating no significant health risk. This study demonstrates that modified PG can meet engineering performance requirements while possessing favorable environmental safety, thereby providing theoretical basis and technical support for its large-scale application in road construction.