Debris flows have caused serious human casualties and economic losses in the regions strongly affected by the Ms8.0 Wenchuan earthquake of 2008. Debris flow mitigation and risk assessment is a key issue for reconstruction.The existing methods of inundation simulation are based on historical disasters and have no power of prediction.The rain-flood method can not yield detailed flow hydrograph and does not meet the need of inundation simulation. In this paper,the process of water flow was studied by using the Arc-SCS model combined with hydraulic method,and then the debris flow runoff process was calculated using the empirical formula combining the result from Arc-SCS.The peak discharge and runoff duration served as input of inundation simulation. Then,the dangerous area is predicted using kinematic wave method and Manning equation.Taking the debris flow in Huashiban gully in Beichuan County,Sichuan Province,China on 24 Sep.2008 as example,the peak discharge of water flow and debris flow were calculated as 35.52 m3·s-1 and 215.66 m3·s-,with error of 4.15%compared to the measured values.The simulated area of debris-flow deposition was 161,500 m2,vs.the measured area of 144,097 m2,in error of 81.75%.The simulated maximum depth was 12.3 m,consistent with the real maximum depth between 10 and 15 m according to the field survey.The minor error is mainly due to the flow impact on buildings and variations in cross-section configuration.The present methodology can be applied to predict debris flow magnitude and evaluate its risk in other watersheds inthe earthquake area. Debris flows have caused serious human casualties and economic losses in the regions strongly affected by the Ms 8.0 Wenchuan earthquake of 2008. Debris flow mitigation and risk assessment is a key issue for reconstruction. The existing methods of inundation simulation are based on historical disasters and have no power of prediction. The rain-flood method can not yield a detailed flow hydrograph and does not meet the need of inundation simulation. In this paper, the process of water flow was studied by using the Arc-SCS model combined with hydraulic method, and then the debris flow runoff process was calculated using the empirical formula combining the result from Arc-SCS. peak discharge and runoff duration served as input of inundation simulation. Then, the dangerous area is predicted using kinematic wave method and Manning equation. the debris flow in Huashiban gully in Beichuan County, Sichuan Province, China on 24 Sep.2008 as example, the peak discharge of water flow and debris flow were calculated as 35.52 m3 · s-1 and 215.66 m3 · s-, with an error of 4.15% compared to the measured values. The simulated area of ​​debris-flow deposition was 161,500 m2, vs. the measured area of ​​144,097 m2, in error of 81.75%. The simulated maximum depth was 12.3 m, consistent with the real maximum depth between 10 and 15 m according to the field survey. Minor error is mainly due to the flow impact on buildings and variations in cross-section configuration. present methodology can be applied to prediction debris flow magnitude and evaluate its risk in other watersheds inthe earthquake area.