In early morning of Aug 8th, 2010, the rain-triggered tremendous debris flows broke out simultaneously at the Sanyanyu ravine and Luojiayu ravine, which locate in the north part of Zhouqu County town. The debris flow is the most severe event of the same kind of disasters in the past sixty years in China, which caused great losses of people’s lives and properties. Based on field investigation, remote sensing image interpretation and analysis of local climatological data, the local topographical conditions, active tectonic movement, massive debris source and torrential rains were the main formation causes which induced the catastrophic debris flows. Moreover, detailed geological surveys were carried out following the disaster, the other geological potential hazard sites were found out, and the geological and seismic hazard assessment has been put into practice. At last, scientific and appropriate countermeasures have been suggested to prevent and mitigate the extraordinarily serious debris flow.