The ferric oxide (Fe3O4)-impregnated magnetic mulberry branch biochar (MBC) was prepared, and a long-term incubation trial was conducted to assess its efficiency in the remediation of As-contaminated soil. The changes in soil physicochemical properties [pH, dissolved organic carbon (DOC), soil organic matter (SOM), and electrical conductivity (EC)], soil nutrient elements, soil enzymes, soil bacterial community structure, and biodiversity at four different doses of MBC (1 %, 3 %, 5 %, and 7 %, wt%) across three aging periods (30, 90, and 180 d) were revealed. The results showed that the residual state of As increased by 161.4 % after 180 day of incubation with 5 % MBC. Spearman correlation and random forest model analyses revealed that available nitrogen had the highest contribution to the arsenic passivation process in soil. The addition of 5 % MBC significantly altered the bacterial abundance and composition in the soil. Actinobacteria, Proteobacteria, Chloroflexi, Bacteroidetes, and Acidobacteriota were identified as the predominant bacteria at the phylum level in all treated groups. The visualizations of soil bacterial co-occurrence networks over different passivate periods indicated more cooperative mutualism and complex interactions from day 30–180. After MBC was added to the soil, the arsenic toxicity was reduced with increasing aging time through bacterial community succession and co-occurrence networks. Overall, MBC facilitated the conversion of available arsenic by influencing the physicochemical factors and microbial community structure of the soil, thus reducing arsenic toxicity. MBC is a promising material for remediating arsenic-contaminated paddy soils.