<p>Fluctuations in reservoir water levels frequently induce deformation of deposits, which may lead to catastrophic landslides. This study investigates the deformation mechanism of Tonglin deposit in Xiangjiaba Reservoir area under the influence of reservoir water level fluctuations. The deformation mechanism of the deposit was comprehensively analyzed through engineering geological investigations, surface and deep deformation monitoring, and pore water pressure loading–unloading creep (PLC) tests on soil samples. Surface cracks and deformation results indicate that the deposit is divided into eastern and western regions. In the eastern region, only the front edge exhibits cracking, with minor deformation primarily characterized by bank collapse. In contrast, the western region exhibits cracking at the front, middle, and rear edges. Surface displacement persists during high reservoir water levels, but the deep displacement curve show no abrupt changes, indicating that the deposit has not formed a continuous sliding surface. PLC test results indicate that soil deformation is predominantly governed by volumetric strain, with deviatoric strain accumulating progressively. After the PLC process, the peak stress ratio of soil samples shows a slight increase. Comprehensive analysis suggests that the deformation in western region of Tonglin deposit is mainly caused by soil creep induced by pore pressure changes due to reservoir water level fluctuations. Although rainfall contributes as a loading factor, the soil stress state is remains under the critical state line, suggesting that the deposit is unlikely to form a continuous sliding surface.</p>