The erosion of debris flows on the material source will affect the movement and impact of the debris flow. This paper adopted the coupled SPH-DEM-FEM to establish a complex dynamic model of the particle-fluid-erosion-structure of the debris flow and to assess the impact of erosion and sedimentation and analyse the dynamic response of the retaining structure of the debris flow. The strain-softening model was adopted to simulate the transformation of the debris flow body from the solid state to the transition state and finally into the liquid state. The coupled numerical analysis completely reproduces the debris flow erosion test, fitting the debris flow shape and thickness profile well. The impact process of the debris flow, the impact height behind the retaining dam, the deposition thickness, and the debris flow dynamic response significantly influence both with and without considering the effects of erosion. The results of this study are similar to existing literature and data, with the numerical analysis being consistent with the physical simulation tests in the existing literature, verifying the applicability of the SPH-DEM-FEM coupling analysis for assessing the debris flow impact retaining structures of erosion and sedimentation. The study also found that the dynamic response considering the debris flow impact and the retaining structures of erosion and sedimentation is more pronounced than when not considering erosion and sedimentation. Coupled numerical analysis can assess the potential effect of erosion and sedimentation, making a significant contribution to the assessment of the impact of debris flow and the design of retaining structures.