<p>Landslides influenced by rainfall and groundwater seepage often exist in a saturated–unsaturated state, making them susceptible to destabilisation. While the Discrete Element Method (DEM) is capable of predicting landslide failure, it does not currently account for the effects of saturated–unsaturated seepage on rock-soil masses. To address this, we use the theory of saturated–unsaturated seepage to investigate how water is conducted between particles within a landslide body. We then establish a relationship between macroscopic permeability and interparticle microconductivity and derive a discrete element equation for saturated–unsaturated seepage. Based on this, a DEM-based model that incorporates saturated–unsaturated seepage, hydrodynamic effects on the stress field and water-induced softening of the landslide body will be developed. This model aims to accurately predict the deformation evolution and ultimate collapse of a landslide due to rainfall and groundwater level change. To validate our approach, we apply the model to simulate the complete deformation process leading to the disintegration of the Tanjiahe landslide in the Three Gorges Reservoir area, China, under the influence of rainfall and reservoir water level changes. The results demonstrate that the intense rainfall penetrated deeply into the middle sliding mass, resulting in an increase in seepage forces near the sliding surface, a reduction in overall shear strength, and a loss of support in the middle sliding mass, which exerted pressure on the front sliding mass. Furthermore, the drop in reservoir water level led to a notable deformation of the front sliding resistance section, ultimately leading to landslide failure. Our method effectively predicts landslide deformation and instability under saturated–unsaturated seepage conditions.</p>