Abstract: The study on the landslide moving and river-blocking mechanism is the starting point of the studies of the landslide river-blocking disaster chain.Many landslides,avalanches and other geological hazards occurred during the "5.12" Wenchuan earthquake, 2008.And most of these hazards happened in the high mountain gorge areas.The moving distance of the landslide was very short,and has the characteristics of "high speed and short route","river-blocking" and so on.Taking Tangjiashan landslide which is the most dangerous river-blocking hazards happened during the earthquake as an example,the 3D instability process of the landslide is reconstructed by using dynamic finite element method based on detailed geologic investigation results of the landslide area.With the analytical results,the forming process and river-blocking mechanism of Tangjiashan landslide can be divided into four phases:(1)earthquake triggering and progressive failure of the rock mass; (2)landslide body failure and sliding in high speed; (3)impacting the opposite slope,disintegrated,blocking the river and the landslide dam formed; (4)density under the impacting.At about 30s after the earthquake the average velocity of Tangjiashan landslide body reaches its maximum value,about 22m 穝-1; and at about 65s,the movement of the landslide reaches to the stabile statues.