Abstract: The mechanical response of the foundation interface in the Ningxia silty fine sand stratum under dynamic loading remains poorly understood. This study systematically conducted interface shear tests using a large-scale multifunctional interface shear apparatus, complemented by particle image velocimetry (PIV) and the discrete element method software PFC3D. The investigation reveals the strength evolution, shear band propagation characteristics, and particle contact reorganization mechanisms of the structure–silty fine sand interface. This research has significant engineering implications for the dynamic design and safety assessment of piles and other foundations subjected to traffic loading in the Ningxia region. Macroscopic experimental results indicate that increasing interface roughness significantly enhances the interlocking, retention, and occlusion of silty fine sand particles by the structural surface grooves, leading to a marked increase in peak shear stress and in the shear displacement required to reach the peak. Greater roughness intensifies particle rearrangement during shearing, further compresses interparticle voids, and substantially increases cumulative shear contraction. When the loading frequency ranges from 0.01 to 1.00 Hz, both the shear stress amplitude and peak strength gradually decrease with increasing frequency, as particles have sufficient time to complete rolling, interlocking failure, and local rearrangement. As the frequency enters the 1.00–2.00 Hz range, the rate of change slows markedly, and the curves tend to flatten, while the valley strength is only slightly affected. Throughout the tests, the dynamic shear stress–shear displacement curves consistently fluctuate between the two static curves corresponding to the upper and lower limits of the stress amplitude, staying closer to the lower-limit curve. Increasing both the initial normal stress and the dynamic stress amplitude enhances the interface shear resistance. When the initial normal stress increases from 20 kPa to 100 kPa, the valley shear strength increases from 18.60 kPa to 65.10 kPa, and the peak shear strength increases from 23.17 kPa to 70.83 kPa. Under dynamic loading, the peak interface friction angle is approximately 25.21°, corresponding to a reduction of about 20% compared to static conditions, while the valley interface friction angle is approximately 33.48°, which is comparable to the static value. Based on these findings, it is recommended that for traffic loading in the 0.01–1.00 Hz frequency range, the foundation bearing capacity be adopted as 0.80–0.85 times the static test value, and that in dynamic design, the peak interface friction angle be taken as 25–26° and the valley interface friction angle as 33–34°. PIV-based displacement and strain field analyses demonstrate that as shear displacement increases, the shear band propagates upward from the structural interface in an arc-shaped form and eventually stabilizes. The strain field is dominated by compressive strain, consistent with the vertical shear-contraction deformation behavior of the soil during shearing. A localized tensile strain zone appears on the left side of the shear box, while a higher compressive strain level is observed on the right side, primarily attributable to the confining effect of the shear box sidewall and the difference in particle movement velocities induced by the ploughing action of the structure, exhibiting pronounced asymmetric deformation characteristics. Discrete element numerical simulations further reveal the intrinsic mechanisms underlying the interface response. As the test proceeds, the force chain paths near the interface deviate from simple straight-line patterns to curved and branched configurations. The number of green contact force chains increases significantly, and a small number of red-yellow force chains also emerge. Concurrently, some force chains undergo breakage and reorganization, reflecting the microscopic–mesoscopic processes of particle relative sliding and rearrangement. Porosity evolution exhibits a “rapid-then-slow” decreasing trend, with fluctuations becoming more pronounced as the stress amplitude increases, indicating that higher stress amplitudes enhance interparticle interlocking and dislocation, promote the filling of interparticle voids by finer particles, and form a denser structural system. The coordination number increases rapidly at first and then stabilizes with increasing shear displacement, and a higher stress amplitude increases the number of effective interparticle contacts, thereby enhancing the interfacial bearing capacity. This study enriches the understanding of the interfacial mechanical response of Ningxia silty fine sand under dynamic loading, providing a quantitative basis and theoretical support for the dynamic design, bearing capacity assessment, and long-term stability analysis of foundation structures in silty fine sand strata in this region.