<p>The interface between soil-rock mixture and concrete often undergoes dynamic loads, necessitating an in-depth study of the cyclic shear characteristics of the soil-rock mixture-concrete interface. A series of cyclic direct shear tests were conducted on the soil-rock mixture-concrete interface using a large-scale indoor direct shear apparatus. The influence of fines content (25%, 50%, 75%) on the stress–strain response and volumetric behavior of the soil-rock mixture-concrete interface was analyzed. A Discrete Element Method (DEM) model was established based on the results of the large-scale indoor direct shear tests to analyze micro-shear characteristics such as contact, particle rotation, displacement, force chain, and fabric anisotropy. The results show that the soil-rock mixture-concrete interface exhibits a shear hardening trend. At 25% fines content, the interface exhibits the highest cyclic shear stress, and the volumetric change of the specimen is proportional to the fine content. As the number of cyclic shear cycles increases, the average force chain strength shows a decreasing trend, and the force chain angle distribution at low fines content differs markedly from the higher-fines cases. The lower the fines content, the more intense the movement of the particles inside the soil-rock mixture. As the number of cycles increases, the rate of change of particle displacement and angle gradually decreases. The principal direction of anisotropy of the normal contact force and tangential contact force between particles deviates more significantly under low fine content, and the anisotropy coefficient decreases with the increase in the number of cycles.</p>