In this study, the calcium-iron composite desulfurized with high activity, large pore size, and high sulfur capacity was successfully developed by adopting hydroxyl-rich iron sludge as the structure and reaction additives, investigating the effects of different contents of iron sludge and anions on the desulfurization activity, aiming to address the key technical problems such as poor reactivity and low sulfur capacity of the fixed bed calcium-based particle desulfurized for flue gas with low exhaust emissions, while completing the utilization of red mud. The desulfurization performance test results showed that the calcium-iron dual component desulfurization agent exhibited a strong cooperating desulfurization effect, and the desulfurization performance was significantly higher than that of Ca(OH)2 or FeOOH individually, in which the 30wt% hydroxy iron oxide (FeOOH) desulfurization agent had the optimal sulfur capacity and better than the reported desulfurization agents of the same type, up to 106 mg SO2/g. N2 physisorption desorption, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, hydrogen temperature-programmed reduction, and thermogravimetric analysis characterization studied reveal that the structural basis of CaO desulfurizer synergistic desulfurization is the introduction of high specific surface area iron sludge during the hydroxylation process of CaO, which generated a highly reactive CaFeO3Cl·5H2O interfacial phase and inhibited the growth of Ca(OH)2 grains. In addition, the rapid evaporation and expansion of internal water during lime hydroxylation results in the formation of a porous, high specific surface area (67.36 m2/g) fluffy particle structure, exposing more abundant SO2 adsorption and reaction active sites, and the pore channels. Water film and active Fe3+ formed by the decomposition of FeOOH component of calcium-iron desulfurized during the desulfurization process effectively promoted the diffusion, adsorption, and oxidation of SO2, further enhancing the reaction and removal of SO2. This study is expected to be able to provide a cheap and convenient purification material and process for the desulfurization and purification of small and medium-sized boilers or bulk exhaust flue gas.