Abstract: "5·12" Wenchuan earthquake caused a high-speed landslide on the Tangjiashan mountain to block the Tongkou river to form the Tangjiashan dammed lake. After a normal discharge on June 10,2008,there were potential hazards of stability loss of remnant Tangjashan mountain body and debris flows in Dashui and Xiaoshui ditches on the right bank of Tangjashan barrier dam. Through an on-the-spot investigation,due to the effect of the earthquake collapses and landslides developed in Dashui,Xiaoshui and Wuming ditches to form extremely rich loose sources,having all conditions of slope and valley debris flows. That become one of the potential serious dangers of geological disasters after the Tangjiashan dammed lake had been successfully discharged and controlled the normal use of the Tangjiashan dammed lake. "6·14" and "9·24" debris flows,two large-scale debris flows occurring respectively on June 14 and September 24,2008,made the Tongkou river blocked respectively for 4 and 17 h. Based on identifying the distribution and stability of loose sources in the three ditches on the right bank of the Tangjiashan barrier dam and importantly by combining the developmental situation of "9·24" debris flow,the developmental and dynamic features of the three ditches,especially the river-blocking situation at discharge spout of the Tangjiashan barrier dam and breaking mode when the three ditches break out debris flows at the same time under different rain frequencies,were analyzed systematically. The research result shows that the possibility that debris fans break once completely is very small after debris flows block the river,and the breaking mode is that firstly flood flows over the edge of debris fans,secondly crushes and scours the left of the barrier body,finally rushes out debris fans gradually. This mode provides a guidance and reference for the research on debris flow in the disaster areas after the Wenchuan earthquake.