In recent years, Korea has often experienced various natural disasters due to localized heavy rain or typhoon. Human and/or property damage by debris flow has been increasing, especially in mountain regions. Numerous studies on debris flow such as the origin of debris flow, analysis of debris flow, areal prediction of debris-flow occurrence is carried out in order to prevent debris-flow damage. This study characterized the impact force of debris flow as function of time by using model experiment and numerical simulation. PFC<sup>3D</sup> program based on DEM(District Elemental Method) was applied for the numerical analysis. The objective of this investigation is to evaluate the shock power of debris flow and to improve the design criteria for protecting structures against debris-flow.