There exists a large-scale geological anomaly with a huge thickness on the left abutment of some dam on Jinsha Rive, which is composed of cracked rockmass, clay on its surface and broken stones. For understanding the essence of the geological phenomenon, a gradual recognition process is as follows: a rocky slope → a fault broken rock belt → a toppling deformation rockmass as unloading → a landslide body. The landslide is regarded as the final recognition. It belongs to the rock-cut landslide, formed in the late Pleistocene. Its volume is about 8 000 × 10 _⁴ cubic meters. Its front is covered by a deep laminated overburden of Jinsha River bed, its integral stability is good, but the impact of its existence on dam site arrangement of the dam is rather great.