Seawater flue gas desulfurization (SFGD) is a promising process for coal-burning power plants located along the coast. The aim of this work was to study the seawater outflow characteristics, especially the relationship between absorbed S(IV) and pH in the SFGD process. The equilibrium concentration of SO2 in seawater was studied under different seawater usages and SO2 concentrations in the flue gas. The experimental results showed that approximately 25% of absorbed S(IV) was oxidized to S(Vi) by surplus oxygen in the flue gas during the absorption process. An equilibrium model was established to describe the relationship between absorbed SO2 and pH. The integral desulfurization efficiency of the absorption system was analyzed, and the results showed that desulfurization efficiency can reach up to 90%.