Evaluation of swelling behavior is of great importance for designing structures in expansive soils areas. Due to infiltration, the expansive soil produces swelling deformation in both vertical and horizontal directions, and the swelling pressure occurs if the swelling deformation was restrained that influences the stability of surrounding structures. In order to reveal the causes of anisotropy behavior of expansive soil in the process of wetting and swelling, from the microscopic point of view, this paper observed the microstructure of expansive soil from Baise with medium swelling potential and Zhijiang with weak swelling potential respectively, and statistically analyzed the orientated distribution of clay mineral particles in expansive soil by image processing technique. Meanwhile, from macroscopic aspect, variations of swelling strain with swelling pressure were measured under lateral confining condition via the developed 2D dilatometer and the modified specimen preparation method. The results show that the microstructure of expansive soil is flaky and stacked layer by layer. When the expansive soil is of natural loose state, the clay mineral particles are randomly oriented and aggregated; After compaction, clay mineral particles tend to be horizontally oriented due to the anisotropic stress. The higher the dry density, the denser the soil sample, and the more obvious its horizontal orientation. Under lateral confined condition, significant swelling differences are observed between the vertical and horizontal direction. The greater the dry density, the stronger the swelling capacity, and the more obvious the difference. However, for clay mineral particles, the swelling direction is perpendicular to their long axis, so the highly horizontal orientation is the reason why expansive soil shows swelling anisotropy behavior in macroscopic view. The swelling anisotropy is influenced by dry density and swelling ability, which is difficult to estimate, therefore it is suggested to measure the swelling principle in two directions in engineering practice.