Abstract: For shield tunnels constructed in gap-graded sandy soils are subject to a significant risk of segment joint leakage during operation. Seepage flow may transport fine soil particles away from the coarse-grained skeleton into the tunnel interior, resulting in suffusion. However, the developmental mechanism of suffusion and its influence on ground stability and tunnel structure remain unclear. In this study, a self-developed visualized test system for tunnel suffusion was employed, coupled with dynamic monitoring of water and soil pressures and quantitative measurement of soil loss, to investigate the characteristics and impacts of the entire suffusion process. A gap-graded sand with 35% fines content was selected. The leakage hole diameter was set to less than twice the coarse particle size, and a constant hydraulic head of 20 kPa was applied to induce suffusion. Test results show that the soil loss rate reaches a peak at the initial stage and gradually decreases until particle migration ceases, while the seepage discharge continuously increases. The suffusion initiates near the leakage hole and progressively expands to form a U-shaped erosion zone, accompanied by stratum deformation and surface settlement. After suffusion onset, circumferential loads around the tunnel decrease, with approximately 70% reduction observed above the tunnel due to severe fines loss, which may lead to tunnel uplift and lateral convergence deformations.