Through field investigation and according to the topography and landform characteristics, structural characteristics of slope, deformation characteristics of slope and deposits characteristics on riverbank, we conclude that the ancient landslide slid down with a high speed after failure, then hit the hill on the opposite bank and blocked river. After a long time of fluvio-lacustrine deposit, the barrier dam overflowed broke down with stepwise scour and the debris were still left on the left and right side of the river. Restoring the original terrain and using general limit equilibrium method, we analyze that the landslide is inducted by earthquake. To further research the formation of blocking river and deformation failure mechanism of the landslide, we employ discrete element method to simulate the landslide process. Results reveal that under natural circumstance, relaxed-tension cracks in shallow surface of the slope developed along the bedding plane and preferred gently-inclined plane; while under earthquake condition, tensioned fracture occurred inside the slope and shear failure happened along the gently-inclined structural plane. The slope experienced the following stages (1) shearing deformation happened in the leading edge and vibration-tension cracks occured in the back edge of the slope; (2) sliding plane penetrated through and the slope slid in high speed; (3) the landslide scraped the riverbed and ploughed the hill on the opposite bank; (4) the leading edge of landslide uplifted for impediment, the back edge stood and glided, and the middle part blocked the river; (5) the landslide self-stabilized after earthquake. The slope has an obvious amplification effect under the action of seismic dynamic load.