An expansive soil slope of a high-speed railway under construction in Yunnan suffered a local collapse after a heavy rainfall. The expansive soil on this slope surface is prone to slip and collapse under the action of rainfall. The analysis of water sensitivity reveals the attenuation law of the shear strength of the expansive soil. With the increase in the water content, the shear strength of the expansive soil decreases significantly. Wet–dry cycles also significantly lower the shear strength of the expansive soil. In addition, a high-density electrical method test is performed on the expansive soil slope to determine the structural characteristics of the internal soil. The test results indicate that the internal soil of the slope exhibits poor mechanical properties and contains fissures and cracks, which are also the factors that caused the landslide. The finite element software, ABAQUS, is used to analyze the impact of rainfall on the expansive soil slope. The results reveal that the soil at a depth of approximately 2 m below the surface reaches complete saturation after 24 h of rainfall, and the plastic failure zone is mainly distributed on the surface and toe of the slope. Based on the electrical test results, the slope soil material is classified into saturated, fissured, and non-fissured soil zones. The stability analysis results reveal that the sliding surface is circular and in the shallow layer of the slope, and the sliding depth is 2–3 m. Although the depth of the landslide mass is not large, almost the entire slope surface soil undergoes plastic failure, resulting in a large area of slippage.