【Objective】The coexistence of cadmium (Cd) and arsenic (As) in soil and water has emerged as a critical global environmental concern due to the significant risks it poses to human health through the food chain. To address this pressing issue, a novel silicon-iron modified biochar (CMSMB) was developed using a co-precipitation-physical mixing method.【Method】The study aimed to comprehensively investigate the remediation capabilities and underlying mechanisms of CMSMB through a series of batch experiments and soil incubation trials in environments contaminated by both Cd and As.【Result】In batch experiments, CMSMB exhibited an impressive maximum adsorption capacity of 272.73 and 17.59 mg?g-1 for Cd(Ⅱ) and As(Ⅲ), respectively. The adsorption processes on the CMSMB surface were intricate, involving a simultaneous interplay of antagonistic and synergistic interactions, and the relative strengths of these interactions were found to be controlled by the concentrations of Cd(Ⅱ) and As(Ⅲ) in the solution. The antagonistic effect primarily originated from the competitive binding of Cd(Ⅱ) and As(Ⅲ) to hydroxyl and aromatic rings. Conversely, the synergistic effect relied on electrostatic adsorption, Cd-As co-precipitation, and the formation of ternary surface complexes. Soil incubation experiments conducted over 20 days revealed significant positive outcomes. The application of CMSMB led to a substantial increase in soil pH and dissolved organic carbon (DOC) content. Consequently, there was a noteworthy decrease (ranging from 64.86% to 74.25%) in the concentration of available Cd in the soil. These changes were attributed to the impact of electrostatic adsorption, precipitation, and complexation resulting from the intricate interplay between CMSMB and alterations in the soil physicochemical properties. However, in the short-term soil incubation, CMSMB exhibited a negligible influence on the bioavailability of As in the soil. The concentration of bioavailable As showed only a slight decline with increasing incubation time which suggests that the remediation effect of CMSMB on As in co-contaminated soils may require a longer duration for observable impacts.【Conclusion】In summary, CMSMB emerges as a potent environmental agent with remarkable efficacy in remediating water contaminated by Cd(Ⅱ) and As(Ⅲ) co-contamination. Furthermore, it demonstrates the ability to passivate Cd in co-contaminated soils, leading to a substantial reduction in the bioavailable Cd. However, its influence on the bioavailability of As in the soil during short-term application appears to be limited. CMSMB demonstrates applicability in the remediation of farmland soils and wastewater contaminated with cadmium and arsenic, found in sources such as mining tailings and agricultural irrigation. However, its long-term remediation capacity, encompassing migration, transformation, and microbiological mechanisms, requires further in-depth exploration and validation.