The aerosol emission from the ammonia-based wet flue gas desulfurization (WFGD) system was investigated in an industrial device and in the lab-scale experimental system. The aerosol formation processes in the WFGD system were simplified into the entrainment mechanism and the heterogeneous reaction mechanism. The aerosol formation properties were studied individually under the two mechanisms. The influences of some desulfurization parameters were also explored. The results indicated that the aerosol emission increased significantly after the ammonia-based WFGD system. The aerosols were mainly in the submicron range in terms of the numerical concentration. For the mass concentration, however, the particles were mostly micron ones. The micron particles were mainly generated by the entrainment mechanism, in which condition the aerosol formation increased with the elevation of the flue gas superficial velocity and the desulfurization solution concentration. In contrast, the heterogeneous reaction mechanism was not only the primary source of the submicron particles but also the major cause of the aerosol increase after the WFGD system. When the NH3 concentration was changed to alter the NH3-to-SO2 molar ratio (N/S), the heterogeneous reactions were boosted by a greater N/S and a lower reaction temperature. When the temperature increased or the N/S decreased, the submicron part of the generated aerosols declined obviously, but the micron part changed little. Accordingly, increasing appropriately the ambient temperature in the scrubber and reducing the ammonia escaping from the desulfurization solution would effectively lessen the aerosol emission from the ammonia-based WFGD system.