Abstract: Biochar (BC) is a porous and carbon-rich material that has been widely used in treatment of heavy metal contaminated water and soil. However, biochar is not effective in stabilizing As(III) in soil due to the electrostatic repulsion between biochar and anion group As. In this study, the iron-manganese oxide modified biochar (FMO-BC) is prepared for remediation of As(III)-contaminated soil. The method includes oxidizing of As(III) into less mobile As(V) by manganese oxide, stabilizing As(V) by iron oxide, and reducing the aggregation of iron-manganese oxide (FMO) by biochar support. For contaminated soil with an As content of 1300 mg/kg, the remediation effectiveness and mechanisms are investigated through synthetic precipitation leaching procedure (SPLP), bioavailability test, pH measurement, sequential extraction procedure, and a series of spectroscopic/microscopic analyses. The results show that FMO-BC is significantly more effective in remediating As(III)-contaminated soil than FMO. At a 7% FMO-BC dosage, the leaching concentration of As decreased to 0.456 mg/L, and the stabilization efficiency reached 98.6%. After BC support, more active sites are obtained for FMO to react with As and therefore the bioavailability of As is reduced. The neutralizing actions of FMO with biochar avoids a significant increase in soil pH, thereby reduces the As leaching associated with high pH. Results of the sequential extraction procedure show that a lot of As is transformed into stable forms after the FMO-BC remediation, thus reducing the environmental risks of As. Spectroscopic/microscopic investigations indicate that 86.9% of As(III) on its surface was oxidized to As(V) by FMO-BC. During the FMO-BC treatment, oxidation, adsorption, complexation, and precipitation reactions are the main mechanisms involved in As stabilization. Remediation of As(III)-contaminated soil using FMO-BC can avoid soil acidification due to the use of FMO and is therefore environmentally friendly and has potential application.