The weak mechanical properties of weak interlayers are crucial for controlling landslide deformation and failure under water level fluctuation. The instability and failure of landslides in reservoirs can lead to unpredictable consequences. In this study, the reservoir bank landslide with a weak interlayer was selected as the research subject. The material composition, structural characteristics, mechanical properties, and permeability of the landslide were determined through field investigations and tests. Additionally, a physical model test was conducted to explore the groundwater variation rules and deformation failure modes of landslides with weak interlayers under different water level fluctuation rates. The results indicate that due to the low permeability of the interlayer, there was a significant lag in monitoring data such as pore water pressure within the interlayer under the same water level fluctuation rate. At the same point, the faster the water level fluctuation rate, the greater the degree of lag. The deformation and failure mode of landslide with weak interlayer under reservoir water level fluctuation can be summarized as the following five stages: slope toe erosion stage, cracks on slope surface and interlayer stage, micro-collapse of slope toes and crack expansion of slope surface and interlayer stage, local micro-collapse of slope toe and crack penetration of slope body stage, crack development leads to landslide of slope body stage. This study provides theoretical support for prevention and control of landslides with weak interlayers in the gravel soils of reservoirs.