<p>Stabilizing piles are widely used in landslide governance, yet their interaction with multi-sliding zones reservoir landslides (MSZRL) remains poorly understood, often leading to unexpected failures. Specifically, the mechanism by which piles influence the force transmission path between adjacent slide masses during reservoir water level (RWL) fluctuations has not been fully elucidated. This study investigates the coupled deformation evolution of MSZRL reinforced by stabilizing piles, using a 3D fluid–solid coupling numerical model based on the prototype of the Majiagou MSZRL in the Three Gorges Reservoir area. The results reveal a critical “bridging effect” of improperly anchored piles: rather than stabilizing the slope, the piles transfer the residual sliding force from the active shallow slide mass to the metastable middle slide mass. This alters the stress distribution, causing a concentration of shear stress that triggers the penetration of shallow (S1) and middle (S2) slip surfaces, thereby accelerating the failure of the deeper strata. Furthermore, by analyzing the evolution of displacement, seepage fields, and unbalanced thrust under varying pile depths, this study classifies the deformation of pile-reinforced MSZRL into four distinct modes: shallow sliding (no piles), middle sliding (short piles), deep sliding (long piles), and minimal sliding (ultra-deep piles). These findings provide new theoretical insights into the interlayer shear coupling mechanism and offer practical guidelines for optimizing pile design in complex multi-layered reservoir slopes.</p>