The deposit body on the left bank of dam at a hydropower station in Southwest China affects the safety of dam, which volume is about 2 million m3. It spreads along the SWW direction like a bugle with a clear boundary, which main material composition is made up of rubble and silty clay. Based on the investigation and research about the basic characteristics and the pattern of deformation and failure, the formation mechanism and influencing factors are analyzed in this article. The results of this research will provide powerful basis to the stability evaluation.