To deepen the understanding for the seepage failure mechanism of earth rock junctions for crossing-dyke sluices, a testing apparatus was designed to accomplish the indoor simulation for the seepage failure process of the junctions between sluice side wall and soil mass. Then, the failure characteristics of contact scouring and its development were analyzed with the clay mass fractions of 4. 6%, 12. 3% and 22. 6%. The effects of soil properties, hydraulic gradient and compactness of the contact zone on seepage failure were studied. The results show that the typical contact scouring process can be divided into three stages, i. e. , stable seepage, piping formation development, and erosion failure. The higher the clay content and compactness, the longer duration will be for stable and failure stages with stronger erosion resistance. There is a significant inverse relationship between erosion and clay mass fraction, hydraulic gradient and compactness. The compactness effect on soil erosion resistance is significant under weak hydraulic gradient. However, with the increase of hydraulic gradient, soil properties play a leading role. At the initial stage of failure, the loss of fine soil particles shows different regularities under the comprehensive effect of soil properties, compactness and hydraulic gradient.