The maximum runout amount is an important factor in debris flow protection engineering design and

hazard assessment. In the Wenchuan earthquake affected region, available methods are not doing well in the calculation of the potential runout amount of debris flow, because the sediment volume of debris flow basin has changed greatly, and the runout amount-frequency relationship differs greatly from the past. Regress analysis on the disaster area reveals that most of the debris flows in clustering debris flow events are of soil-mechanical type, and there exist a strong power-function correlation between sediment volume and maximum runout amount for the triggering rainstorm of 20-year return period. Volume and properties of sediment have a decisive influence on maximum debris flow magnitude. Different amount of debris flows for different debris flow watershed is decided by the hydrological forces from the rainfall processes. The micro-topography change resulted in a higher peak discharge and runout amount than predicted. The larger the frequency of triggering rainfall and the more sediment volume ina debris flow watershed, the more the runout amount. There is an obvious linear relationship between areas of rectangle watersheds and corresponding maximum runout amounts, while the linear relationship is undetectable for fan-shaped watersheds.