At most areas in northeast margin of Sichuan Basin in southwestern China, the hilly landform and red beds with low-angle dip are the typical characteristics. An indiscernible ancient landslide was found in this area along the river bank, and people realized the whole town was built on the large ancient landslide deposit. In case the landslide ranout, catastrophic land deformation would destroy the town. Based on detailed survey of the landslide, the evidences for the ancient landslide were revealed. By ways of geological analysis, the failure mechanism of landslide was concluded that the reasons for the ancient landslide was due to the internal cause and the external cause. By comparison of the mineral component characteristics of landslide deposit and the bedrock, the original landform of the slope was reconstructed. In the end, a geologic sketch map was drawn to explain the deformation process of the ancient landslide. Moreover, the formation reason for steep rock wall on the boundary of landslide was revealed.