With the development of industrial process precision, fluid-solid particle two-phase flow systems have been widely used in the chemical industry, energy, and other fields. According to the different mediums in the continuous phase, the fluid-solid two-phase flow can be divided into gas-solid and liquid-solid two-phase flow, in which the accurate measurement of particle size and concentration characteristic parameters are very important for process control and efficiency optimization. To address the problem that the classical ultrasonic attenuation model is difficult to adapt to different medium types and cannot cover the full wavelength range, this article analyzes the advantages and disadvantages of the classical theoretical model based on the numerical simulation method of thermal viscous acoustic physical field. The coupled superposition method of the McClements model and the BLBL model are adopted to achieve the comprehensive characterization of different attenuation mechanisms. An ultrasonic attenuation McBL model suitable for low-concentration fluid-solid two-phase flow in the full wavelength region is formulated. Combined with the particle swarm optimization algorithm, the simultaneous inversion of particle size and concentration is achieved. The simulated attenuation coefficient is taken as the inversion input to preliminarily verify the applicability of the model. Compared with the particle parameters of the simulation model, the simulation results show that the errors of the average particle size and concentration obtained by inversion are both within ±15%. In addition, an ultrasonic attenuation experimental device is set up to measure the ultrasonic attenuation coefficient of the fluid-solid particle two-phase flow employing a dual-frequency transmission method, and the McBL model combined with particle swarm optimization algorithm is used to invert the experimental attenuation coefficient. Compared with the measurement results of the laser particle size analyzer, the experimental results show that the average particle size error of the particles is within ±10%. This further verifies that the McBL model combined with the particle swarm optimization algorithm has high accuracy in the inversion of characteristic parameters in the full wavelength region of fluid-solid two-phase flow. This research is of great significance for the online precise measurement of characteristic parameters of flu-solid two-phase flow.