To further explore the expansion and shrinkage mechanism of expansive soil of the canal slope in the first phase of Northern Xinjiang Water Supply Project, the free expansion rate, loaded expansive soil rate and shrinkage test of the expansive soil under three different circulation modes were carried out. The law of expansion and shrinkage characteristics was analyzed from a macro perspective, and the physical mechanism was revealed from a micro perspective by scanning electron microscope test.The results show that the expansion deformation of expansive soil can be divided into three stages, including rapid, uniform and stable expansion with the time increase. The expansion change is mainly concentrated in the rapid expansion stage. The larger the overburden load, the smaller the loaded expansion rate. The overburden pressure inhibits the water absorption and expansion of expansive soil. With the increase of the cycles, the unloaded expansion rate, the loaded expansion rate and the linear shrinkage rate all show a decreasing trend, and gradually tends to a stable value after the fifth cycle. Among them, the dry-wet-freeze-thaw cycle decreases the most, followed by the dry-wet cycle, and the freeze-thaw cycle is the smallest.In terms of the microscopic mechanism, after the sample is cycled, the porosity and the total number of particles in the soil increase gradually, while micro indexes such as the total area of particles decrease gradually.The size and content of clay particles are the main factors affecting the expansion and shrinkage. The cycle effect affects the changes of the size, content and arrangement of clay particles. Among them, the total area of dry-wet-freeze-thaw cycles has the greatest change, and the expansion and shrinkage of expansive soil samples have the greatest change. The freeze-thaw cycle has the least influence on the microstructure, and the expansion and shrinkage of expansive soil samples have the least change.