Abstract: The concentration and particle size of limestone slurry 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, and obtain the model coefficient matrix correcting for the nonlinear effect of concentration. By combining the ultrasonic attenuation spectra and differential evolution algorithm, the particle size distribution of limestone was inverted. The influence of changes in slurry concentration on the resultant particle size distribution was assessed. An online test system was developed to continuously monitor the limestone slurry concentration and particle size for 24 hours in a coal-fired power plant. 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 particle 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.