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