Abstract: In order to reveal the seismic damage characteristic and evolution mechanism of slopes under strong earthquakes, the geological conditions and seismic cracks of three typical seismic cracked mountains in Songpinggou valley, Diexi Town, Maoxian County, Sichuan Province were investigated. The main findings were as follows: ① the propagation of seismic cracks on the slope was mainly related to the characteristics of structural planes, and the seismic cracks were often triggered and propagated along joints. ② The combination characteristics of seismic cracks controlled the instability mode of the cracked slope. ③ In the process of strong earthquake, compression crack, tension crack, and tension crack were mainly triggered by compression-tension loadings. Under the dynamic shear loading, tension echelon and compression torsion echelon cracks were induced. ④ The seismic cracks induced by the earthquakes accelerated the failure of the slopes. The seismic cracks caused by seismic loading in strong earthquake mountainous area are the key factor of slope instability.