Abstract: Limestone slurry concentration and particle size are key factors affecting the efficiency of flue gas desulfurization (FGD) in coal-fired power plants, and their on-line measurement is very important for the slurrying process. The Harker & Temple coupled phase combined with Bouguer–Lambert–Beer Law scattering ultrasonic model was used to describe the attenuation of ultrasonic waves in the slurry. Experiments were carried out to establish the fitting relationship between relative sound velocity and slurry volume concentration based on the ultrasonic phase velocity spectral analysis, to obtain the model coefficient matrix correcting for the nonlinear effect of concentration. By combining the ultrasonic attenuation spectra, the differential evolution algorithm was employed to invert the particle size distribution of limestone, and to assess the influence of changes in slurry concentration on the resultant particle size distribution. An online test system was developed to continuously monitor the limestone slurry concentration and particle size in a coal-fired power plant for 24 hours. The results show that the slurry volume concentration in the stabilized section of the mill is maintained at 18.78±2.47%, while the concentration in the regulated section ranges from 10.95% to 32.95%; the median diameter of the slurry volume during the testing process ranges from 16.73 to 21.88 μm, and the error between this method and the image method is less than 9%. Thus, the ultrasonic method has been used to monitor the concentration and particle size distribution of desulfurization slurry in real time, which provides support for operators to adjust the mill status, achieving the purpose of energy-saving operation and reducing desulfurization costs.