On September 14，2019，a large loess landslide occurred in Tongwei County，Dingxi City， Gansu Province. Through on-site investigation of the loess landslide and drone aerial survey，the topography，geomorphology，hydrogeology and other conditions that gave birth to the landslide were ascertained，and the morphological characteristics，structural characteristics and movement patterns of the landslide body were studied in-depth，revealing the Tongwei landslide Disaster mechanism. The high-density electrical method is used to detect the stratum structure，thickness of the sliding body， groundwater distribution and spatial distribution of the landslide area. Combining the finite element method and Morgenstem-Price method to calculate the stability of the slope，the distribution of the maximum shear strain and the variation of slope stability with rainfall duration are obtained. The results show that：(1)the shape of the Tongwei landslide is a chair-like shape，which is divided into three typical failure areas，forming a large number of vertical ridges，and the thickness of the loess sliding is approximately 8~50 m；(2)the Tongwei landslide belongs to the "traction-push" type of bedding landslide，and the movement mode is "pull toe instability traction-hindered sliding in the middle partinstability shift in the rear part"；(3)the groundwater is mostly fissure karst water，and the geological structure is not obvious. The geological structure activities such as early earthquakes have a great influence on the stratum；(4)the stability of loess slope decreases continuously with rainfall duration，and the maximum shear strain region is mainly distributed in the middle and upper part of the slope，and develops from mudstone contact to slope surface；(5)rainfall is the most direct factor triggering the Tongwei landslide，and the development of gullies，river erosion and agricultural production activities are important disaster-generating conditions