Abstract: This paper took the soil samples in Qian'an area of Jilin Province as the research object and tested them with the dispersion test and scanning electron microscope(SEM)test. The fissures of soil samples were quantitatively treated by MATLAB image processing software and CIAS fissures quantitative analysis program. The dispersion variation characteristics and micro mechanism of original and modified soil samples under the freeze-thaw cycle condition were explored and the relationship between soil sample dispersion and fissure parameters was analyzed. The results show that: (1)The dispersion of the samples without alum modification increases with the increase of freeze-thaw cycles, while the dispersion of the samples with alum modification does not change under the condition of limited 20 freeze-thaw cycles. (2)The structural unit of soil samples is broken under the action of freeze-thaw cycle, which increases the fine-grained content in the soil sample, thus enhancing the dispersion of the soil. Meantime, the newly formed fissures cut soil into blocks of different sizes and shapes, which further increases the dispersion of the soil. (3)Among the fissure parameters, the fractal dimension of fissures has a good correlation with the grade value of soil dispersion, followed by the fissures rate. (4)The aluminum ion in alum replaces the sodium ion on the surface of the soil particles, increases the attraction between the soil particles, thus reduces the dispersion of the soil, and the dispersion of the sample is less affected by the freeze-thaw cycle conditions. Therefore, alum can be used as a good modifier for the treatment of the seasonally frozen differentiated bulk soil.