Dissolved Organic Matter (DOM) is easily adsorbed and transformed by soil minerals and is an important redox-active component of soil and sediment. However, the effects of the molecular weight of DOM on the interface between MnO2 and DOM remain unclear. Herein, fulvic acid (FA) from peat was size-fractionated into four molecular weight fractions (FA>10kDa, FA5–10kDa, FA3–5kDa, and FA<3kDa) and then reacted with δ-MnO2 in this study. The affinity of FA for MnO2 varied significantly with different molecular weights, and large molecular weight FA was more easily adsorbed by MnO2. After 30 h of reaction, the highest mineralization rate was for FA>10kDa (42.39 %), followed by FA5–10kDa (28.65 %), FA3–5kDa (25.58 %), and FA<3kDa (20.37 %), consistent with the results of adsorption. The stronger reducing ability of the large molecular weight fraction of FA to MnO2 was mainly attributed to hydrophobic functional groups, promoting adsorption by MnO2 and the exposure of more active sites. The main active species involved in the mineralization of FA were •OH and Mn4+ through the quenching experiment. Our findings confirm that the large molecular weight fractions of FA play a crucial part in the adsorption and redox reactions of MnO2. These results may help evaluate the performance of different molecular characteristics of FA in the biogeochemical cycles of MnO2 in the soil environment. Display omitted •FA from peat was size-fractionated into four molecular weight fractions.•The affinity of FA for MnO2 varied significantly with different molecular weights.•Large molecular weight fraction of FA were more easily adsorbed and mineralizated.•The main active species involved in the mineralization of FA were •OH and Mn4+.