Abstract: In April 2017, a slope in Pingquan City, Hebei Province, China showed signs of large-scale sliding deformation after multiple excavations. Although the excavation revealed that the slope underwent toppling deformation, the surrounding rock strata are gently inclined, the terrain is flat, and the slope height and gradient do not meet the typical conditions for toppling deformation. Based on this unusual topping-deformation phenomenon, this study analyzed the evolution process and mechanism of the original slope topping-deformation of Nandongzi landslide trough field geological investigations and drilling methods. Deformation monitoring and Flac 3D numerical simulation were used to study the deformation and instability mechanism of tipping deposits. The results show that Nandongzi landslide is a south-north overturning deformed body, and the Nanlaodong fault crosses and ends in the landslide area, forming the right boundary of the landslide. Nandongzi landslide mainly experienced four evolutionary stages: initial deformation, compressive bending, gravity collapse, and sliding failure stage. Adjacent faults and soft and hard interbedded rock mass are the main factors causing the toppling deformation of the original slope; the whole toppling deformation is formed by the accumulation of rock compression, bending, and toppling. Nandongzi landslide is a sliding landslide, with rainfall and multi-stage excavation being the primary factors contributing to sliding failure of the accumulation body. These findings have significant practical and reference value for the treatment and stability evaluation of similar dumping bodies.