Abstract: The secondary geological disasters triggered by "4·20" Lushan strong earthquake were dominated by rockfalls rather than landslides, and their scales were not large. However, there was a large-scale landslide in Damiao Village, Laochang Township, Tianquan County, the only large-scale landslide in the disaster area. There were about 35.2×104 and 42.0×104 m3 strongly weathered dolomitic limestone sliding out quickly from high places along both sides of thin ridge in Dayanbeng mountain, moving along grooves on both sides with a high speed, then sliding about 504 and 740 m respectively along the left and right ditches, hitting with their main channels and climbing about 35 and 26 m respectively in the left bank of Chunjianwo ditch and the right bank of Gangoutou main ditch. Afterwards they slid about 763 and 409 m respectively and intersected 409 m away from the impact point in Gangoutou ditch, then they slid about 223 m because of gentle slopes to form two small dammed lakes with volumes of 8 000 and 600 m3, causing no casualties and property loss because of no inhabitants along the ditch. On the basis of detailed geological survey and combining with field mapping, prospecting and other means, the basic features of Dayanbeng landslide were researched thoroughly, and the causes of the landslide were analysed preliminarily. The research result shows that the thin ridge, weathered dolomitic limestone and favorable structures are the basic conditions of the landslide, and vibration effect of "4·20" Lushan earthquake is the direct cause of the landslide.