Abstract: The biochar (BC) has been widely used in treatment of heavy metals-contaminated water and soil. However, it is not effective in stabilizing As(Ⅲ) in soils due to the electrostatic repulsion between the biochar and the anion group As. The iron-manganese oxides-modified biochar (FMO-BC) is proposed for remediation of As(Ⅲ)-contaminated soils. The manganese oxide can oxidize As(Ⅲ) into less mobile As(Ⅴ), and the iron oxide can stabilize As(Ⅴ), while the biochar loading can avoid the aggregation of iron-manganese oxides (FMO). The remediation effectiveness and mechanisms are investigated through the synthetic precipitation leaching procedure, bioavailability tests, sequential extraction procedure and a series of spectroscopic/microscopic analyses. The results show that the remediation of FMO-BC is significantly more effective than FMO. At a 7% FMO-BC dosage, the stabilization efficiency reaches 98.6%. After the biochar support, more active sites are obtained for FMO to react with As, and therefore the bioavailability is reduced. The neutralizing actions of that of FMO with biochar avoids a significant increase in soil pH, thereby reduces the As leaching associated with high pH. A lot of As is transformed into stable forms after the FMO-BC remediation, thus reducing the environmental risks of As. 86.9% of As(Ⅲ) is oxidized to As(Ⅴ) by FMO-BC. The oxidation, adsorption, complexation and precipitation reactions are the main mechanisms involved in As stabilization.