Expansive soils are widely distributed and the landslides happened in these area cause significant economic losses. The sliding surface records crucial information about landslides, providing insight into the mechanisms behind their occurrences. This research delves into the microstructural features of landslide surfaces in expansive soils. Various sliding surfaces were created through direct shear tests under different water contents (10%, 20%, 30%) and normal stresses (50 kPa, 100 kPa, 200 kPa, 400 Pa). Scanning electron microscopy (SEM) was used to observe the surfaces and based on their structural morphology and formation mechanisms, we categorized the surfaces’ structure into scratch, tension crack, and residual pore. Four parameters, porosity, average pore area, probability entropy, and area probability distribution index, were calculated and used to quantitatively characterized the surface structures. By correlating field landslide surface features with geological survey data, it was inferred that the triggering moisture content was 25.3–26.8% at least, the sliding surface was situated 10 to 12 meters deep within the slope, and multiple sliding events occurred. The water infiltration from the slope top and precipitation along fractures coupled with rising irrigation water levels that saturated the slope base ultimately leading to fracture layer breakthrough and slope failure.