With the increasing attention on SO3 emission in coal-fired power plants, the removal of SO3 acid mist in a limestone-gypsum wet flue gas desulfurization (WFGD) system was investigated. The generation properties of SO3 acid mist were analyzed with an experimental system, and the effects of flue gas properties and technologies for ultralow emission on the SO3 removal in WFGD systems were investigated with a pilot plant. The results revealed that the sizes of SO3 acid aerosols generated by homogeneous nucleation were principally less than 0.1 mu m, and it had a positive impact on the coagulation and growth of fly ash particles via the collision. When the gas temperature at the inlet of the scrubber went up, the removal efficiency of the WFGD system on SO3 acid mist was decreased, and the decline was more obvious as the temperature fell below the dew point of the acid. Higher concentrations of SO3 and fly ash before desulfurization were beneficial for the improvement of SO(3 )removal. Furthermore, technologies for ultralow emission improved the removal during the desulfurization process. The removal efficiencies in different WFGD systems were increased by 18-28%, and the enhancement effect of these technologies was shown in order as the addition of tower component < single-scrubber with dual-cycle < dual-scrubber with dual-cycle.