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Abstract: Soil-bentonite (SB) cutoff walls are commonly used as barriers in polluted areas. The embedded part of an SB wall in an aquitard is crucial for its performance. In this study, a centrifuge modeling test was carried out to investigate the effect of contact between the key and the aquitard on the migration behavior of contaminants within a sand-bentonite cutoff wall. The centrifuge was accelerated to 100g and maintained in-flight for 36 h, equivalent to 41 years of transport time in the prototype. Results showed that the contaminant concentration within the SB wall was higher downstream than in the middle in the thickness direction, and deeper regions exhibited a greater concentration than shallower ones. This concentration distribution indicated that contaminants were transported along the interface between the SB wall and the aquitard, bypassing the base of the SB wall to reach the downstream aquifer rapidly. An improved numerical simulation considering preferential interface migration was performed, which agreed with the centrifuge test results. The simulation results indicated that preferential interface migration, as a defect, significantly accelerated the speed of contaminant migration, reducing the breakthrough time of the SB wall to 1/9 of that without preferential interface migration.