Based on the mechanism whereby cations loaded onto modified biochar can displace Na⁺ in saline-alkali soils via ion exchange, this study used corn-stover biochar as the carrier and employed ball-milling to composite it with flue-gas desulfurization gypsum (DG), single superphosphate (SSP), and poly-aluminum ferric sulfate (PAFC), thereby preparing three modified biochars (BCDG, BCSSP, and BCPAFC) and systematically evaluating their amelioration effects and underlying mechanisms in soda saline-alkali soil. The results showed that ball-milling modification markedly altered the physicochemical properties of biochar; among the three, BCPAFC exhibited a highly porous structure, abundant amorphous metal hydroxyl complexes, and higher polarity and hydrophilicity. Incubation experiments indicated that BCPAFC effectively regulated soil alkalinity, stabilizing soil pH (initially 8.25) within a near-neutral range (6.49-6.88), while maintaining a relatively high electrical conductivity (0.50-0.55 mS/cm). Meanwhile, it increased Ca²⁺ and Mg²⁺ concentrations in soil solution, promoted sustained Na⁺ release and leaching, and reduced sodium adsorption ratio (SAR) to 2.73-4.24. In addition, after 12 days of incubation, BCPAFC decreased exchangeable sodium (ENa⁺) by 31.7%, increased cation exchange capacity (CEC) by 86.4%, and reduced exchangeable sodium percentage (ESP) by 63.4%, demonstrating a pronounced advantage in reshaping soil exchangeable-ion equilibrium. Microbial community analysis further revealed that BCPAFC enhanced bacterial α-diversity, enriched salt-tolerant taxa such as Actinobacteriota and Pseudomonadota (Proteobacteria), and increased alkaline phosphatase and urease activities. Correlation analysis suggested that soil pH, base cations, and carbon-nitrogen nutrients were the primary environmental drivers governing microbial community succession. Overall, BCPAFC enables rapid amelioration and ecological function recovery of soda saline-alkali soil through the synergistic action of multiple mechanisms, including ion exchange, specific adsorption, structural regulation, and microbial activation, providing important theoretical and technical support for the development of biochar-based soil remediation materials.