Dissolved organic matter (DOM) in the natural environment has a significant effect on the biogeochemical cycles of iron (Fe), such as adsorption and reduction reaction. However, the roles of the molecular weight (MW) fractions of DOM extracted from plants in adsorption onto Fe minerals coupled with the Fe reduction process are incompletely characterized. In this study, the effects of DOM (extracted from Carex cinerascens) with different MW fractions, labeled as low molecular DOM (L-DOM), medium molecular DOM (M-DOM), and high molecular DOM (H-DOM), respectively, on the adsorption onto Fe minerals and the electrons transfer capacity (ETC) during the Fe reduction process is investigated. The oxygen-rich and highly aromatic H-DOM exhibited stronger adsorption affinity onto Fe minerals, such as ferrihydrite, a poorly crystalline mineral, than other DOM fractions. The X-ray photoelectron spectra and the Fourier transform infrared spectra revealed that the functional groups of the oxygen-rich DOM, such as CO bonds and OCO bonds, preferentially adsorbed onto the Fe minerals. In addition, the cyclic voltammograms (CV) and the chronoamperometry (CA) measurements showed that H-DOM significantly enhanced the Fe reduction to its maximum extent, indicating that the efficiency of DOM on Fe reduction was strongly related to the ETC. Therefore, our findings confirm that H-DOM plays a key arole in the adsorption onto Fe minerals and Fe redox reaction. The results of this study may help evaluate the performance of the different MW of DOM from plants on the biogeochemical cycles of Fe in the soil environment.