Abstract: The expansive soil canal slope of the first phase project of water supply in northern Xinjiang will experience several processes of water supply, of water interruption, of water freezing and melting every year, which will lead to sliding failure of canal slope. To further explore the sliding failure mechanism of expansive soil canal slope, the physical mechanism of expansive soil degradation is analyzed from the macro-fine-micro three angles through the fracture test, shear test and micro test of expansive soil. The results show that: Expansive soil after the drying-wetting-freezing-thawing cycles, the fracture rate, the cracks number, and the total cracks length of expansive soil show a sharp upward trend, and then the speed slows down and finally stabilizes. Due to the problems of single measurement of existing indexes, upon existing fracture indexes, a new fracture evaluation index Q is proposed by the number of intersection points and cracks, which can comprehensively evaluate the degree of fracture development. Q increases with the increase of cycles, and then slows down and finally tends to be stable. After different times of drying-wetting-freezing-thawing cycles, the cohesion of expansive soil decreases with the increase of cycles, and the internal friction angle is basically at a stable value. The shear strength is mainly related to the cohesion. The relationship curve between cohesion and fracture parameter Q varies linearly, and the cohesion decreases with the increase of Q. In terms of microscopic mechanism, it is found that the soil microstructure is significantly affected by the drying-wetting-freezing-thawing cycles by micro-electron microscope scanning. The soil microstructure is gradually broken, the total particles area is reduced, the pore volume is increased, and the micro pores are continuously connected to form new cracks. The grey correlation degree between fracture parameters Q and microscopic parameters is greater than 0.67, and the total number and total particles area are the main influencing factors. The generation of cracks makes the interaction force between soil particles gradually decrease, resulting in smaller cohesion.