Abstract: The scanning electron microscope (ESEM) is used to obtain the scanning image of the microstructure of expansive soil under different soaking times. Based on the professional image processing software Image-Pro Plus (IPP), the ESEM images of the expansive soil are processed and analyzed. The apparent pore ratio, fractal dimension and composition of expansive soil under different soaking times are studied. The results show that the apparent pore ratio of the expansive soil increases gradually with the increasing soaking time, and its growth rate decreases gradually. The small island method is used to study the particle size and pore size of soil samples. The results indicate that the particles and pore of the expansive soil have obvious fractal characteristics, and the fractal dimension ranges from 1.1 to 1.3. With the gradual increase of the soaking time, the fractal dimension of the particles decreases, and the fractal dimension of the pores increases, and the two trend are opposite. The soil particles and the pore are classified as four groups of large (>5 μm), medium (2-5 μm), small (1-2 μm) and micro (<1 μm). Evolution of various types of particles and pores under different soaking times are also analyzed.