<p>Particle size is a pivotal factor influencing the mechanical behavior at the pile-soil interface. Leveraging a self-designed large-scale cyclic shear apparatus for soil-structure interfaces, an exhaustive investigation was conducted into the variation patterns of shear stress, normal displacement, shear stiffness, and damping ratio at the pile-soil interface under cyclic loading conditions. Various factors, including particle size, shear displacement amplitude, and normal stress, were taken into account. The findings reveal that interface shear stress escalates with an increase in displacement amplitude. Specifically, the shear stress–shear displacement hysteresis loop adopts a elliptical shape at small displacement amplitudes and transitions to a parallelogram shape at larger amplitudes. Both the enclosed area of the hysteresis loop and the shear stress augment with an increase in particle size. The normalized interface peak shear stress gradually shows hardening characteristics with the increase of the number of cycles, and is positively correlated with the displacement amplitude, particle size and normal stress. Initially, the soil undergoes complete shear-induced contraction. As the experiment advances, it exhibits alternating patterns of shear contraction, dilation, and re-contraction, although an overall trend of shear-induced volumetric contraction persists. The vertical displacement of the soil is more pronounced with smaller particle sizes and larger displacement amplitudes. With an increase in cycle count, the interface shear stiffness gradually rises and attains stability, whereas the interface damping ratio decreases. These shifts are notably influenced by displacement amplitude and particle size.</p>