Abstract: Mega-earthquakes often occurred in loess hilly region of China. Earthquake-triggered landslides and other geological hazards can be the main reason for a large number of casualties and property losses. The 1718 great Tongwei earthquake in northwest China caused more than 300 large-scaled landslides near the epicenter. The effects led to the movement of the northern mountains located in the meizoseismal area of Pan'an Town(the old Yongning Town),to the south, resulting in the total burial of Yongning town, with a historical record of over 30,000 deaths. Previous studies agree that the earthquake-induced landslides in the area are loess landslides, which slid along ancient terrain surfaces. Through detailed field investigations, the distribution features, the types of disasters and failure mode are studied. Taking the largest landslide-Weijiawan landslide as a case study and with the help of engineering geological drillings on the sliding mass, deep structure characteristics of the landslide are analyzed and the mechanics mechanism are discussed. The results indicate that the failure surface of the huge earthquake-induced landslide is located in the deep mudstone layer, which reveals that these landslides are loess-mudstone composite landslides. These new findings can help to further unravel the failure mechanisms of these earthquake-induced landslides that are recorded in several literatures. Meanwhile, the present results and conclusions can provide insights and knowledge on early recognition of earthquake-triggered landslides in northwest China towns, especially in the surrounding valley which shares similar geological conditions to the landslides studied in this paper.