Abstract: The collapsible loess in Yan′an New Area was chosen to carry out the immersion test and the microstructure images of loess with different immersion periods were obtained by scanning electron microscope. The image processing software Image Pro Plus (IPP) was used to extract the microstructure parameters of loess, and the microstructure of loess was quantitatively analyzed with the fractal theory. The results show that the collapsibility of soil decreases during the immersion process. The arrangement and form distribution of soil particles change regularly. Also, both the probability distribution index of pore area and the fractal dimension of particles showed a decreasing trend, the roundness of particles first decreased and then increased, and the alignment of particles changed in direction with the time of immersion. Furthermore, the influence of water immersion on the microstructures of loess is time-sensitive. In the initial stage, the soil is affected by softening of clay minerals and the loss of soluble salts, and the natural structure gradually becomes unstable under the disturbance of water; As the immersion continues, the skeleton structure is damaged, and the particles deflect, slip, and recombine; In the equilibrium stage, the particles form new structures and become
denser, the large and medium pores decrease, and the soil structure gradually stabilizes. In this paper, the characteristics of loess structures under the action of flooding are studied from both macroscopic and microscopic perspectives, which provides an experimental and scientific basis for engineering problems in collapsible loess areas.

Key words: loess, immersion action, microstructure, quantitative analysis, collapsibility