A gentle bedding slope (16° dip angle) failure at Sanxicun (SXC) village in Dujiangyan city was triggered by heavy rainfall in 2013. The landslide (composing of sandstone and mudstone strata) has a sliding distance of up to 1200 m and caused 166 deaths. After this failure event, a detailed field survey and a series of laboratory tests were carried out by us to investigate the progressive failure mechanism of the slope. The results revealed that the slope deformation could be traced back to the Ming Dynasty in China (400 years before) at least. A sliding block with a width of 330 m and a length of 240 m detached its original position, resulting in a depression belt with 10 m in width and 25 m in depth at the rear of the sliding block. On the scarp, a large number of cracks were widely observed not only in the sandstone but also in the mudstone. The field evidence revealed that the depression belt and cracked mudstone had provided convenience for water infiltration for a long time. The shake table test results showed that these cracks could be initiated and propagated in the slope during earthquakes. Especially, seismic stresses amplifications were induced in the mudstone to degrade this layer obviously. Besides, Direct shear tests of cracked mudstone with different immersion time were carried out. It was showed that the shear strength of mudstone decreased rapidly in the initial immersion and then tends to be stable gradually. Finally, numerical calculation indicated that the hydrostatic pressure in the depression belt and uplift pressure in the basal layer generated by the strongest rainwater in history (537.4 mm daily) caused the kilometer- slide of the block in 2013. We concluded that the catastrophic failure of the SXC landslide triggered by the rainwater in 2013 strongly depended on the pre- existing deformation and damage caused by historical earthquakes and rainwaters.