A deeply excavated expansive soil slope of a canal section with high groundwater level in the Nanyang basin of a water transfer project was investigated. According to the data of geological survey and the data during the construction and operation periods, the appearance disaster characteristics of the canal slope after deformation were summarized. The depth and the geometry of the potential shear deformation zone of the deformed body were described. The deformation mechanism was comprehensively analyzed from the internal and external factors such as engineering geology and hydrogeology