<p>The comprehensive development of seepage failure cannot be effectively observed using conventional testing methods, which significantly hinders our understanding of the mechanisms underlying subsurface erosion. This study presents an experimental investigation aimed at elucidating the seepage failure process in unstable sand, utilizing a self-developed seepage test device based on transparent soil visualization techniques. Transparent soil made from fused quartz sand was employed to simulate natural sand, while a mixture of 15# white oil and n-dodecane served as the pore fluid. Utilizing particle image velocimetry software PIVlab, we investigated the influence of fine particle content and seepage velocity on the seepage failure mechanism of poorly graded sand. The test results indicated that the characteristics of seepage failure varied at different locations within the specimen, with the dominant flow primarily concentrated in the centroid section, ultimately leading to the deformation failure of the soil skeleton. As the fine particle content increased, the threshold for initiating seepage failure also rose. Furthermore, varying levels of fine particle content resulted in different forms of seepage failure, while seepage velocity significantly impacted the particle transport pathways.</p>