Abstract: Tanggudong landslide which is huge lies among the pre-selected upper, middle and lower dam of Lenggu hydropower station. It has a decisive effect on the selection of hydropower station dam site and construction and layout. Studying formation mechanism of landslide has a very important influence on analyzing deformation and evolutionary of similar slopes in this river. On the basis of analyzing geo-environmental conditions and characteristics of landslide, the author analyzes formation mechanism of landslide by means of bottom friction test method and discrete element numerical calculation. The results show:Tanggudong landslide is a giant and rock landslide, of which the deformation and failure model is slipping-cracking. It cuts off layers and moves along the ladder-like sliding surface which is composed of the steeply inclined outside joints and gently inclined outside joints in the strongly weathered layers. Instability and failure of these front slopes firstly occur, which is then followed by the continuous deformation of the upper slopes. Consequently, instability and failure of the whole upper slope in the rear part ultimately occurs.