The emission and formation characteristics of aerosols were studied during the ammonia-based wet flue gas desulfurization (WFGD) process. The particle characteristics before and after desulfurization were measured in a power plant. The aerosol formation characteristics and effects of operation parameters were investigated with an experimental system. The results indicated that the aerosol emission was related to the type of ammonia-based desulfurization technology and the particle morphology and elements changed significantly during desulfurization. Aerosols formed during desulfurization were from the heterogeneous reactions and the entrainment and evaporation of desulfurization liquid. The heterogeneous reactions made greater contribution to the aerosol formation with different components, and smaller aerosols were generated. With the decrease of gas flow velocity and the solid content of desulfurization liquid, the aerosol concentration from the entrainment and the evaporation of the desulfurization liquid decreased. The formation of aerosols through the heterogeneous reactions could be inhibited with the decrease of the pH value of the desulfurization liquid and the inlet flue gas temperature.