Abstract: Cadmium (Cd) and arsenic (As) are highly toxic heavy metals that pose serious risks to human health. This study investigated the synergistic passivation effects of iron-modified biochar derived from different feedstocks (rice straw, pig manure, and leaf litter) on Cd and As in contaminated soil. The physicochemical properties of the modified biochars were characterized using X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), Fourier transform infrared spectroscopy (FTIR), and Brunauer-Emmett-Teller (BET) analysis. The results confirm the successful loading of Fe³⁺ onto the biochar surfaces and show significant increases in specific surface area, micropore surface area, and total pore volume compared with unmodified biochars. A soil incubation experiment demonstrated that all iron-modified biochars effectively reduced the bioavailability of Cd and As. Leaf litter-derived biochar (PIBC) achieved the highest reduction in Cd, decreasing its available content by 70.0% compared with the control (CK). In contrast, rice straw-derived biochar (SIBC) showed the greatest effect on As, reducing its concentration to 0.08 mg·kg^-1 by day 20. Correlation analysis reveal that soil pH, soil organic matter (SOM), and cation exchange capacity (CEC) were significantly associated with Cd and As availability, indicating that iron-modified biochar enhances passivation through alterations in soil physicochemical properties. Overall, this study highlights a sustainable strategy for the utilization of agricultural waste and the remediation of Cd- and As-contaminated soils.