Debris flows with high velocity may have enormous impact forces on obstacles in their flow paths. It is necessary to install protective structures, such as check dams that slow down or interrupt the debris flows from affecting adjacent infrastructure and residential communities. The impact forces of debris flows on check dams are an essential factor in hazard mitigation evaluation and design of check dams. To accurately evaluate the impact force of debris flows on the check dams, a numerical model that takes into account the fluid-solid interactions is needed. In this study, the large deformation analysis, which is the coupled Eulerian-Lagrangian (CEL) technique, was applied to evaluate the behavior of the debris flows and the impact force on the check dams, simultaneously. The numerical method was validated using published data on laboratory experiments. A series of numerical analyses were performed to evaluate the significant influencing factor on the dynamic impact force of debris flows, such as the flow velocity and the thickness of sedimentation. Based on the results of these analyses, it was observed that the dynamic impact force of debris flows on the check dams is significantly dependent on the velocity of the debris flows. In addition, the debris flows are gradually accumulated towards the top of the check dams after the debris flows first contacts the check dams, thereby the position of the dynamic load acting on the check dams is increased.