<p>The disaster of reservoir landslides caused by high groundwater levels has attracted much attention. Among them, old reservoir landslides with saturated slope bottoms are more prone to instability and re-sliding. Using the model test method, an independent seepage device was designed to simulate the whole process in bottom-saturated sandy clay old landslides. In the test, various monitoring methods such as deformation marker points, particle image velocimetry (PIV), and various test sensors were used to analyze the characteristics of instability and failure. The test results indicate that the high groundwater level causes the softening of the bottom clay layer to form a sliding surface, and the plastic flow of the saturated soil layer forms a weak substrate effect, resulting in more staggered cracks in the upper part of the slope, forming a multi-stage sliding surface, sliding section, and sliding mass. Three main failure zones were observed in the main body of the model, a graded sliding zone, a traction zone, and a mudflow zone at the leading edge. Its deformation characteristics mainly include multiple failure modes, such as mudflow failure at the leading edge, graded block traction slip, and local collapse failure. The failure mode is as follows: bottom water level rising, slope bottom saturation, tension crack forming at the trailing edge, local traction creep, water level further rising, graded traction slipping, multistage sliding masses instability, and mudflow zone forming. The research findings can provide insights for the model test research and failure mechanism analysis of old landslides on reservoir banks.</p>