Abstract: With the increase of extreme heavy rainfall events, large-scale slides occurred at many gentle slopes in the piedmont in Nanjing, which is inconsistent with the traditional view that gentle slopes in the piedmont are in favor of the overall stability of the mountains. The analysis of the engineering geological characteristics of the Youzi Mountain landslide shows that it is a typical rainfall-induced landslide preliminarily. At the same time, the sliding mechanism of the gentle slope is studied based on the stratigraphic lithology, geomorphological characteristics, rainfall conditions and engineering activities in the landslide area. The results show that the natural slope of the mountain in Nanjing is of unique stratigraphic structure characteristics: the upper part of the mountain bedrock is bare and the topography is steep; the middle and lower parts are widely covered with the Quaternary unconsolidated sediments, and the slope is gentle, forming a gentle slope in the piedmont. The surface of gentle slope is dominated by fine-grained soil, while the middle and lower layers are dominated by coarse-grained soil, gravel or weathered rockmass, forming a special "binary stratigraphic structure". Under the extreme heavy rainfall, the upper part of the mountain is basically stable, and the bare rockmass fractures are in favor of rainfall infiltration. The middle part of the mountain by precipitation is prone to circular sliding, and the lower part of the gentle slope is easy to form a "temporary confined water", which is prone to landslide under the combination of downslide force of the middle slide body. GeoStudio software is used to calculate and analyze the deformation of the slope. The upper part of the slope first reaches the limit equilibrium state. Since the lower part of the temporary groundwater in the confined aquifer cracked the thinner soil layer in the slope foot, the mountain slipped as a whole, which effectively verifies the formation mechanism of the landslide. The research results are of great significance for the management of similar landslide disaster in Nanjing.