Abstract: Biochar, as a green and sustainable soil stabilization agent, has been widely used in the remediation of heavy metals (HMs) contaminated soil. The migration and transformation of HMs are highly affected by the physicochemical characteristics of soil. Under the change of redox process caused by flooding-drainage intercropping in paddy soils, the fluctuation of physicochemical characteristics of soil will threaten the long-term safety of land utilization. Here, a mulberry stem was selected as feedstock to produce biochar by the in situ "water-fire coupled" method. The derived biochar was noted as SG-BC. The capability and mechanisms of SG-BC adsorbing Cd, Zn, and Pb were investigated. The results show that the maximum adsorption capacities for Pb, Zn, and Cd of SG-BC were 214.39, 41.10, and 25.75 mg·g^-1, respectively. The mechanisms were clarified as surface complexation, precipitation, and cation-π interactions. In addition, a soil microcosm incubation experiment was conducted under redox dynamic conditions to explore the impact of SG-BC on the soil Eh and pH, and the immobilization of HMs. The results show that compared to the control, the addition of SG-BC could increase soil pH, slightly decrease soil Eh, and inhibit the soil HMs release at various degrees during flooding, and the inhibition was more strong during drainage, especially in the early stage; SG-BC decreased the concentrations of Pb, Zn, and Cd in soil solution by 96.69%, 80.77%, and 78.41%, respectively. These results indicate that SG-BC is effective to immobilize Cd, Zn, and Pb in the soil with dynamic redox processes. This study provides theoretical basis for applying biochar to remediate HMs contaminated agricultural soil.