Abstract: Expansive soil is categorized as a special soil that experiences swelling when subjected to water,leading to restrictions on engineering construction. In areas characterized by frequent earthquakes,expansive soil slopes can become vulnerable to water swelling as well as instabilities caused by seismic activity. To reveal the inherent mechanism of expansive soil slope instability in earthquake-prone areas,this study analyzed the expansibility and mechanical properties of expansive soil under multiple dry-wet cycles by conducting unconfined expansion tests,triaxial tests,and microstructural observations. The test results show that the expansion rates after 3,7,10,15,and 20 dry-wet cycles are 4%,7.2%,7.9%,9.1%,and 10%,respectively. An increase in the number of dry-wet cycles causes the expansion rate to increase continuously. Additionally,while the expansion rate increases slowly after long-term water saturation,repeated dry-wet cycles cause a sharp increase in expansion. The pre-dynamic treatment and dry-wet cycles cause the expansive soil to swell with hydration and shrink due to dehydration,resulting in a significant decrease in the shear strength of the expansive soil samples after dry-wet cycles. After 20 dry-wet cycles,the cohesion and internal friction angle of the samples decrease by 41.21% and 36.89%,respectively. Seismic activity causes the formation of micro-cracks in the expansive soil,resulting in greater expansion of pre-dynamically treated samples undergoing dry-wet cycles compared to untreated samples. The deterioration effect from expansion and contraction in strong earthquake areas under the combined action of earthquakes and rainfall causes a sudden drop in expansive soil strength,which in turn induces expansive soil slope instability. Therefore,drainage and anti-seepage measures are particularly important in preventing and controlling expansive soil slopes in earthquake-prone areas.