【Objective】The reductive release of arsenic (As) from paddy soils can be enhanced under waterlogged conditions. This study aimed to investigate the mechanisms of Mn-modified biochar (MBC); with high oxidation and adsorption capacity, on inhibiting As release in the waterlogged paddy soil microcosms and revealing the potential pathways of mediating As transformation and speciation in soil solution and soil. 【Method】This study determined changes in the basic properties of soil solution, soil, and biochar through mixed or separate incubation of biochar and flooded soil. 【Result】The results show that, compared with the control and unmodified biochar, MBC significantly promoted the oxidation of As(III) to As(V) in the soil solution, keeping a low concentration of As(III) (0.02-0.88 mg·L-1) throughout the incubation period. The Mn released from MBC into the soil solution was readily precipitated with As(V). Also, the MBC containing rich pore structure and oxygen-containing functional groups would promote its adsorption for more inorganic arsenic (iAs). MBC showed greater affinity for Fe (3.12 mg·g-1 of Fe was adsorbed on MBC after 28 d), leading to a significant decrease of Fe concentrations in the soil solution (P < 0.05) and enhanced the adsorption of iAs on the solid-phase. The reduction of Mn-oxides on MBC increased the pH (0.08-0.22 pH units) of the soil solution, which further promoted the precipitation of Fe on the solid phase thereby strengthening its adsorption for iAs. As a result, the concentration of iAs extracted from the MBC was 12 times higher than that of the unmodified biochar after incubation of 28 d. The high oxidative properties of MBC inhibited the reductive dissolution of Fe minerals, thereby significantly reducing the soil-available Fe and As concentrations (P < 0.05). This led to the transformation of soil available As to iron-manganese bound and residual fraction As. As a result, the soil available As was stabilized under a lower range of concentrations. 【Conclusion】Generally, the addition of MBC to waterlogged paddy soil can inhibit the release of iAs from the solid phase and promote the transformation of mobile iAs into more stable forms in the soil, resulting in a significant reduction in arsenic mobility and toxicity in waterlogged paddy soil.