Abstract: The water-sediment separation function of retaining dams makes it an important engineering measure for the prevention of debris flow disaster. The loose material accumulated downstream of the retaining dam has gradually become the main material source of the next debris flow. Therefore, it is necessary to analyze the erosion process and the variation characteristics of water content and pore pressure under the influence of retaining dam. and it is of great guiding value to the prevention and forecast of gully debris flow. Taking the debris flow deposition downstream of the second retaining dam of Cutou Gully in Wenchuan County as the research object, using the laboratory physical model test and combined with rainfall data, digital terrain data, and in-situ deposition parameters, this study simulated the erosion process of deposition under the different intensity of rainfall and over-dam flow conditions. The sensors were used to deeply explore water content variation and pore water pressure in the erosion process. Four groups of experiments were carried out in this study, and the experimental factors included four rainfall intensities and four corresponding runoff. It turns out that:(1) Under the influence of retaining dam, the regularity of erosion process is more obvious:splash erosion → surface erosion → headward erosion → lateral cutting erosion → collapse; (2) The infiltration of rainwater in the accumulation body is from shallow to deep, and reaches saturation when the water content reaches about 23%; (3) Under the action of runoff erosion, the pore water pressure increases rapidly before the debris flows starts; (4) The over-dam flow is an important reason for the damage of the deposition. The action mechanism is mainly to change the soil particle skeleton, increase the pore water pressure, reduce the effective stress of the soil, and accelerate the water infiltration. Under the comprehensive action, the failure of the deposition starts from the toe of the slope and then to the upstream and deep. The research results are of great significance for improving the soil erosion characteristics of the deposition and revealing the erosion mechanism under complex hydrodynamic conditions.