Abstract: Expansive deformation of expansive soil slopes caused by rainfall is a typical multi-field coupling problem of unsaturated soils. The instability process of the gradual destruction of rainfall infiltration is investigated in this paper. Based on the saturated-unsaturated seepage theory, the elastoplastic constitutive relationship of expansive soil and strain softening theory, train softening model, FLAC3D secondary development platform and built-in FISH language are adopted, and a multi-field coupled numerical analysis method considering unsaturated seepage, expansion deformation and strain softening is proposed. Combined with engineering examples, this method is used to explore the law of unsaturated seepage, displacement response and progressive failure of expansive soil slopes under rainfall infiltration. The results show that the expansion deformation and strain softening are controlled by the spatiotemporal distribution of unsaturated seepage, which has a significant effect on the slope displacement response process, and it also easily causes the saturated-unsaturated boundary to form a shear stress concentration zone. The progressive failure of the expansive soil slope changes from local failure to overall instability. The plastic failure area of the expansive soil slope first expands into the slope with the change of the suspension-type transient saturation zone. After the rain, it gradually forms a second sliding belt extending from the foot of the slope to the top of the slope, showing the multiple sliding characteristics and backward traction-type failure.