Abstract: The mechanism of SO₄^2- removal enhanced by sodium alginate in the lime softening method was explored in this study, by treating the actual desulfurization wastewater with different combinations of lime milk, polymeric ferric sulfate and sodium alginate, through analyzing the SO₄^2- concentration, pH value, Zeta potential, particle size, surface morphology, functional groups and crystal structure after coagulation. The results showed that compared with the dosing of 7 g/L lime and 40 mg/L polymerized iron sulfate at the same time, compounding with sodium alginate could make the SO₄^2- concentration in the effluent decrease from 3991.15 mg/L to 3238.60 mg/L (mean value), and the pH value increase from 9.66 to 9.69 (mean value). Sodium alginate promoted the aggregation of the colloids through the adsorption charge neutralization and adsorption bridging mechanisms, and formed flocs with a median particle size of 30.41 μm at a dosage of 3 mg/L. By adding sodium alginate into the system, the formed CaSO₄ crystals were more regular, and the crystal clusters were relatively smaller but quite dense, thus enhancing the removal of SO₄^2-. The infrared spectroscopy and X-ray diffraction analysis indicated that, the carboxyl group (—COO^-) of the sodium alginate might facilitate the dissolution of lime by chelating the metal ions such as Ca²⁺, and the crystallization of CaSO₄ was better. The orthogonal experiment results showed that the suitable compound for SO₄^2- removal was composed of 7 g/L of lime, 30 mg/L of polymeric ferric sulfate and 2 mg/L of sodium alginate, and then the removal efficiency was 80.94%.