In recent years, particle size and concentration measurements have been widely studied at home and abroad. An ultrasonic attenuation model based on the Monte Carlo method is extended to a high concentration for online measurement of desulphurization limestone slurry fineness in coal-fired power plants. The attenuation coefficients of the high-concentration Monte Carlo model ( HCMCM) are consistent with experimental measurement values. The model matrix constructed by HC-MCM can be used for the inversion of particle size distribution to reduce the calculation time. The relative deviation between the calculated value by the differential evolutionary algorithm and the set diameter and distribution width is less than 1% . A transmission and reflection dual-mode ultrasonic measurement device and software measurement system have been developed. The transmission sensor and ultrasonic probe are made to be suitable for the fineness measurement of high-concentration slurry, and have completed the 168-hour online measurement experiments relying on a coal-fired power plant. The results show that the trend and value of the attenuation spectra obtained by the ultrasonic transmission and reflection methods are basically the same, and the relative deviation between the volume median diameter DV50 and the measurement results of the laser particle analyzer is less than 8% . Its particle size satisfies the fineness requirements of finished limestone slurry in coal-fired power plants.