Abstract: In order to control the particle size of desulfurization by-product gypsum and improve the quality of desulfurization gypsum, it is necessary to measure the density, concentration and particle size of gypsum slurry quickly and accurately. In this paper a non-destructive desulfurization gypsum slurry particle size measurement method using ultrasonic attenuation spectrum was designed (a.k.a ultrasonic method), and then implemented to examine and compare the contents of six samples of gypsum slurry from different power plants. Based on the ECAH theoretical model, the ultrasonic attenuation spectrum was obtained by Fast Fourier Transform (FFT) of ultrasonic time-domain pulse signal. Different inversion programs were written in MATLAB language. Then by taking advantage of the optimal regularization algorithm (ORT), projection algorithm (Projection) and non-independent genetic algorithm (GA), the gypsum slurry particle size distribution were calculated respectively, and compared with the conventional testing methods. Among them the reliability of the regularization algorithm is verified with the deviation below 20%. The present ultrasonic method for measuring the particle size of desulfurized gypsum slurry can provide reference for the upcoming development of on-line measuring instrument for the particle size distribution of desulfurized gypsum slurry.