In order to clarify the effects of initial soil moisture content and dry density on the expansion process mechanism of purple soil, selected the typical purple soil in the Three Gorges Reservoir area as the research object. Through indoor unloaded expansion test, used two-factor analysis of variance and response surface analysis to investigate the unloaded expansion characteristics under different initial water content (5%, 10%, 15%, 20%, 25%) and soil dry density (1.1g/cm3, 1.3g/cm3, 1.5g/cm3). The results showed that: (1) The variation of the unloaded expansion rate of purple soil with time was obvious, which could be divided into three stages: rapid expansion, slow expansion and stable expansion.(2) In the stage of rapid expansion and slow expansion, the unloaded expansion rate of purple soil increased logarithmically, and the semi-logarithmic function was used to fit the expansion time course of purple soil, the fitting effect was better(R2 ＞0.65). The unloaded swelling rate of purple soil decreased with the increase of initial moisture content, when the dry density was increased, unloaded swelling rate of purple soil increased. The analysis of variance showed that the initial moisture content, dry density and the interaction between the two had a significant effect on the unloaded swelling rate of purple soil (p<0.01). The response surface relationship model could better reflect the relationship between the initial moisture content, dry density of soil and the unloaded expansion rate of purple soil (R2=0.92). (3) The expansion process of purple soil is consistent with the water absorption process, and most of them expand greatly in the initial stage of water absorption. The time required for the water content of purple soil to reach the stable state is shorter than the time required for the soil expansion rate to stabilize. The results can provide scientific basis for sustainable development of agriculture and protection of purple soil resources in the Three Gorges Reservoir area.