Abstract: Seepage erosion is one of the main reasons for the instability and failure of hydraulic engineering. In the process of seepage erosion, the fine particles of the soil are gradually lost, and the permeability of the soil is enhanced, which further affects the loss of particles, and eventually even leads to instability and destruction. The seepage erosion is a typical fluid-solid coupling problem. To explore the interaction between fluid movement and particle loss in the process of seepage erosion from the mesoscopic level, by use of the transparent soil technology, a set of seepage erosion test system based on the double light source PIV/PTV is developed. The fused quartz and solution of CaBr₂ are used as transparent soil and pore fluid. The movement of pore solution and fine particles in the soil during seepage erosion is observed and recorded. The flow velocity of different sections is compared with the macroscopic flow velocity of the sample, and it is found that when the hydraulic gradient is small, the measured flow velocity of the section is larger than the macroscopic flow velocity of the sample. With the increase of hydraulic gradient, the macroscopic velocity of the sample is gradually larger than the measured velocity of the cross-section, and the closer to the center of the sample, the smaller the difference between the macroscopic velocity and the cross-section velocity. At the same time, the fine particles in the sample gradually transitioned from a stable state to move perpendicular to the plane of the water inlet and outlet, and gradually lose. Compared with the calculated results of the critical hydraulic gradient formula for cohesive soil, it is found that the critical hydraulic gradient obtained through the experiment is slightly lower than the theoretical result. However, the regularity reflected by the experimental results is consistent with the traditional test results. It is shown that the test system has high reliability in the meso-simulation of seepage erosion, and has significance for the study of seepage erosion from the meso-level.