The increasing generation of industrial solid wastes, such as red mud and phosphogypsum, poses significant environmental challenges due to their complex chemical compositions and low utilization rates. This study aims to develop an innovative composite material by combining RM and PG, modified with ferric chloride (FeCl₃) and sodium silicate (Na₂SiO₃), to address their environmental risks and enhance their potential for soil and ecological remediation. The modification mechanisms and immobilization of toxic ions were investigated through leaching behavior analysis and advanced microscopic techniques, including BET and XRD. Under the optimal ratio (RM:PG = 7:3), the composite material exhibited excellent performance, with stable pH (8.03), low electrical conductivity (4.89 mS/cm), and significantly reduced concentrations of phosphate (PO₄³⁻: 0.36 mg/L) and fluoride ions (F⁻: 1.34 mg/L), achieving an upgrade from industrial Class II to Class I slag. The modification process increased the specific surface area, optimized pore structure, and enhanced surface activity and structural stability. Pot experiments demonstrated that the modified composite supported normal plant growth, with leachate meeting Grade I wastewater discharge standards. This study not only provides a sustainable approach for the utilization of RM and PG, but also offers valuable insights into the development of eco-friendly materials for soil remediation and ecological restoration, benefiting both the scientific community and environmental management practices.