The investigated large hydropower station is located in the middle reach of Yalong River. During the investigation, it is found that there are 10 extralarge and huge ancient landslides developed in a short river section of 30 km. Survey shows that the landslide group has enormous and unavoidable impact on the proposed project with huge landslide volume of 4 000×10⁴ m³ to 12 000×10⁴ m³and a total length of several kilometers along the river, as well as a maximum depth of over 200 m, and 70% of the landslide developing at the right bank. In this paper, factor analysis shows that the development of landslide group is inevitable as it is closely related to factors such as the valley shape, steep canyon geomorphology, alternatively distributed soft and hard rock layers, and the cutting of partitioned reverse fracture. The formation mechanism can be concluded into 3 categories, namely slidingtension fracturing, slidingbending(buckling), and bendingtension fracture(collapse). Finally, based on the assessment of landslide stability and the influence of dam location on the project, the following conclusion is obtained: as the landslides and dam sites are interspersed, the upstream dam site is the best choice for engineering safety as it can avoid three huge landslides which have signs of reactivation. Corresponding project proposal is also put forward.