Abstract: This study simulated the wet desulfurization process under laboratory conditions and delved into the impact of polymeric iron sulfate impurities on the crystallization of desulfurization gypsum. Experimental results indicated that the addition of polymeric iron sulfate significantly prolonged the crystallization induction time and inhibited the nucleation rate. When the amount of polymeric iron sulfate added reaches 0.4 mL (volume fraction 0.20%), its effect on the nucleation stage gradually weakens, but its effect on the growth stage of gypsum crystals significantly increases. When further increased to 2 mL (volume fraction 0.99%), polymeric ferric sulfate not only inhibited the formation of calcium sulfate in the early stage of the reaction, but also inhibited the formation and growth of gypsum crystal nuclei. Compared to the blank control, when the addition amount is less than or equal to 0.4 mL, the peak of the particle size distribution curve is concentrated. However, when the addition amount reached 2 mL, the peak width increased. Morphological analysis reveals that gypsum crystals are predominantly plate-like with distinct boundaries. With the increase in the amount of polymeric iron sulfate added, there is a gradual increase in granular attachments on the crystal surface, leading to gypsum crystals that appear fragmented and thin, with rough surfaces and significantly reduced quality. In addition to the target component, hemihydrate calcium sulfite is also present in the dihydrate gypsum crystal sample. As the amount of polymeric iron sulfate added increases, the quality of dihydrate gypsum crystals gradually decreases. Further analysis suggests that Fe³⁺ adsorbs on the lattice surface and may enter the lattice, replacing Ca²⁺, leading to an increase in interplanar spacing. Additionally, the addition of polymeric iron sulfate forms Fe—O—S bonds, indicating an interaction between Fe³⁺ and sulfate ions, and a decrease in the binding energy of oxygen.