A novel calcium-based magnetic biochar (Ca-MBC), made by pyrolyzing a mixture of rice straw, iron oxide (Fe₃O₄) and calcium carbonate (CaCO₃), was developed for the remediation of co-pollution of arsenic and cadmium. Characteristics of the material revealed that Fe₃O₄ and CaCO₃ were impregnated on the surface of the biochar. Batch experiments confirmed that the Ca-MBC had a high adsorption capacity for arsenic and cadmium in aqueous systems with maximum adsorption capacities of 6.34 and 10.07 mg g^-1, respectively, and that the adsorption of both metals were pH-dependent from 2 to 12 with an optimal pH of pH 5. The mechanism of co-adsorption of Cd(II) and As(III) were both antagonistic and synergistic. A 160-day soil incubation was conducted to further investigate the remediation efficiency of heavy metals in soil. Following biochar application at the rates: biochar (1%, 2%, 3%) and Ca-MBC (1%, 2%, 3%), soil pH was increased and the bioavailability of cadmium was significantly decreased. The bioavailability of arsenic decreased, compared with the control, as the addition of Ca-MBC increased from 1% to 3% while the biochar treatments significantly increased arsenic bioavailability. These new results provide valuable information for the application of Ca-MBC as a potential material in the treatment of water or soil contaminated with As(III) and Cd(II).