In high earth-rockfill dams, large shear displacement exists in the contact area between the earth core and the concrete cushion at the bottom and on both sides of the banks of a river valley. Exposed to a high hydraulic gradient, seepage erosion is likely to occur in this area. A practical measure is to compact a thin layer of clayey soil with high plasticity such that it lies between the earth core and the concrete cushion. It is crucial to evaluate the capabilities of a clayey soil for this thin layer. A new apparatus modified from a conventional triaxial apparatus is developed to test the seepage characteristics of the clayey soil-structure interface under large relative shear conditions. Compared to the previous device, the new device can achieve a larger relative shear displacement that is closer to the actual condition while providing stress and a seepage state with a simple testing procedure. This apparatus consists of a soil-structure model, a vertical loading system, two back-pressure systems, and a monitoring system. The vertical loading system allows the soil-structure displacement to develop at a constant speed. Two back-pressure systems can simulate the high hydraulic gradient and the consolidation state of the specimen. The monitoring system automatically records the force load, the volume of water outflow, and the volume change in the pressure chamber. Basic tests and shear-seepage tests were conducted using this device to validate its efficiency and reliability. The results indicate that the permeability of the soil-structure interface decreases with the shear displacement over a large range. A finite element (FE) analysis was carried out to reveal the strain-stress and seepage state inside the specimen to explain the seepage mechanism.