Limestone-gypsum wet flue gas desulfurization (WLG-FGD) technology is wildly used to diminish sulfur dioxide emissions in coal-fired power plants, where the particle size of the limestone slurry is a crucial factor closely related to the desulfurization efficiency. The high-concentration Monte Carlo model (HC-MCM) was developed for attenuation spectrum prediction. In numerical simulations with a particle diameter range of 10–70 μm, the differential evolutionary (DE) algorithm was used for retrieving particle sizes with relative deviations that do not exceed 2 % in terms of a given diameter and distribution width. A through-transmission/pulse-echo dual-mode ultrasonic system has been established. The results indicate that the root mean square relative error (RMSRE) of the two modes’ attenuation spectra was 7.03 %. The relative deviations of DV90 are less than 8 % compared to the results obtained through the laser particle analyzer. Moreover, the fineness is within the specifications required for limestone slurry, indicating proper mill performance.