Oxidation of magnesium sulfite (MgSO3) is a crucial step for reclaiming the product in wet magnesia desulfurization processes. Here, for enhancing this reaction, a bimetallic catalyst was developed by loading CoOx and MnOx species on a biomass-derived active carbon (AC) support to minimize the costs and potential environmental risks during catalyst application. The substitution effect of Mn to Co sites was investigated, and a comparison of the catalyst with plain cobalt suggested that the ratio of Co/Mn must be greater than 3. A series of catalyst characterizations was performed to reveal the synergistic effect of Co and Mn in the bimetallic catalyst. The introduction of Mn species not only improved the dispersion of CoOx-MnOx mixed oxide but also generated abundant Co3+ species and surface-adsorbed oxygen, both of which acted as the main active sites for sulfite oxidation. Notably, in the bimetallic catalyst, the presence of Mn4+ species assisted regeneration of Co2+ to Co3+ species, further accelerating sulfite oxidation. Besides, the partial substitution of Co sites by Mn also suppressed the losing of Co species during reaction, favoring to decrease the environmental risk, as well as to save the cost of catalyst.