Arsenic exists ubiquitously in the soil and has been proved to be of significant hazard to human health upon transmission through food chain. Herein, we determined the effects of Fe–Mn–La ternary oxide-biochar composites (FMLBCs) on arsenic (As) fractionation, soil enzyme activities, and microbial communities in arsenic-polluted soils. The results demonstrated that the proportion of non-swappable As fractions reduced and that of the exchangeable As fractions increased with the addition of FMLBCs. Furthermore, the addition of FMLBCs significantly increased the catalase (CAT) activity (P < 0.05), and an increase of 69.2–268% was observed when 2 wt% FMLBCs were added. Supplementation with biochar or FMLBCs increased the relative abundance of Proteobacteria and Acidobacteria and decreased the relative abundance of Firmicutes; moreover, the effect was more obvious as the addition amount of biochar or FMLBCs increased. In addition, the FMLBCs, except for FMLBC₃, increased the content of available phosphorus. Moreover, amendments of FMLBCs led to an increase in the available potassium content by an average of 212%, 113%, and 62.1% in highly polluted soil. Therefore, the FMLBCs affected the physical and chemical properties of soil in different manners. The results suggested that the addition of FMLBCs changed the distribution and increased the immobilization of As in the soil; this could indirectly reduce the risk of the transport of As to rice. The amendment mechanism of FMLBCs may include changes to the physicochemical soil properties and consequently, the soil enzyme activities are affected, which can influence the microbial communities in soils.