Abstract: This paper summarizes the reasons for dam break of tailings reservoir based on some typical cases. The fault tree of dam break of tailings reservoir is constructed by analyzing the destroying form of five factors, dam flooding, dam leakage, loss of dam slope stability, liquefaction of dam body and natural disasters. The major reasons for dam breakage are traced by qualitatively and quantitatively applying the minimum cut sets, minimum path sets and structural importance. Some key preventive measures are put forward.