Abstract: In regions with expansive soil, oil pipelines are affected by rainfall-induced softening, sliding, and hygroscopic swelling of expansive soil slopes, leading to potential deformation and rupture. Such failures could result in severe accidents, including oil and gas leaks or explosions. Using an expansive soil slope in the Nanning Basin as a case study, laboratory drying-wetting cycle tests were conducted to examine the strength degradation and swelling deformation of the soil. Based on field investigations and experimental data, a numerical slope model was developed, incorporating a hygroscopic swelling constitutive model. The finite element fluid-structure interaction method was then applied to simulate the slope 's deformation and failure process. The experimental and numerical results demonstrated that the strength of expansive soil in the study area decreased significantly with increasing drying-wetting cycles, with cohesion and internal friction angle reductions of 59.73% and 30.92%, respectively. During multiple rainfall events, the effective stress of the expansive soil on the slope continuously declined as water content increased, leading to gradual strength degradation under drying-wetting cycles. Sliding failure ultimately occurred after the fifth rainfall. The oil pipeline at the landslide 's front edge was affected by the combined effects of slope softening, sliding, and hygroscopic swelling. The lateral earth pressure behind the pipeline exhibited three stages over time: slow increase, accelerated increase, and eventual stabilization. This progression reflected the pipeline 's initial loading, stress accumulation, and eventual failure due to the coupled effects of slope sliding and hygroscopic swelling. A significant decoupling between earth pressure and displacement was observed near pipeline failure: earth pressure accumulation decelerated, whereas displacement increased abruptly. These phenomena and mechanisms provide a scientific basis for developing deformation and failure prediction models and establishing early-warning thresholds for oil pipelines crossing expansive soil slopes.