Deeply excavated expansive soil canal slopes face the risk of instability during its operation period. This paper presents a case study of a moderately expansive soil canal slope, with a height exceeding 40 m, from a long-stretching water transfer project. The slope stability and the effectiveness of reinforcement measures were assessed. Multiple approaches, including safety monitoring data analysis, non-destructive detection, and numerical simulation analysis, were utilized. The deformation mechanism was evaluated by considering internal factors such as engineering geology and hydrogeology, as well as external factors like rainfall and changes in groundwater levels. The surface modified soil failed to prevent water vapor exchange between the expansive soil and the air. Moreover, during rainy season, precipitation replenished the stagnant water within the upper layer of the canal slope, resulting in a rise in groundwater levels. Conversely, the groundwater levels dropped during dry season. These cycles of drying and wetting led to the development and penetration of fissures in the canal slope. Additionally, creep deformation occurred along the fissure surface of the 2nd to the 4th slopes, influenced by both the fissure surface and the arrangement of anti-sliding piles in the water-passing section. As a result, the local slope stability did not meet specification requirements. The potential sliding surface exhibited characteristics of a combined sliding surface, with a gentle inclination angle at the leading edge and a steep inclination angle at the trailing edge. The potential shear outlet was identified at the first-level berm, although the exact location of the trailing edge of the landslide remains unclear. To enhance slope stability, a combination of drainage ditches and drainage wells proved effective as drainage measures. These findings offer valuable insights for the operation management and reinforcement governance of similar projects.