Numerous debris flow disasters with difficult drainage exist in northern Xinjiang of China. Civil engineering facilities such as provincial highways and gas stations built on alluvial fans at the mountain outlets block the normal discharge of debris flows into the main river, making it impossible to implement drainage measures downstream. Therefore, studying mitigation methods and engineering effects for debris flows in areas with inadequate or difficult drainage conditions is of great significance. This study focuses on the Coxon ditch in Burqin County, Altay Prefecture, Xinjiang, China. At present, a water-intercepting seepage drainage zone has been established downstream of the Coxon ditch. Through two sets of physical model tests under different rainfall intensities, and based on process imaging and monitoring data, we investigate the variation characteristics of soil pressure, pore water pressure, and moisture content during the seepage drainage process of debris flows in drainage-challenged areas. The seepage drainage patterns are summarized, and the underlying mechanisms are revealed. The results show that the drainage process can be divided into five stages: under the condition of 50-year rainfall intensity confluence, which is, channel surface erosion stage, ditch bank erosion stage, seepage discharge stage, seepage deceleration stage, and seepage end stage; under the condition of 100-year rainfall intensity confluence, which is, seepage drainage stage, seepage deceleration stage, water overflow stage, accumulation starting behind the dam stage, and seepage end stage. The soil pressure curve in the upstream channel first rises and then declines, while downstream, soil pressure increases due to sediment deposition. The pore water pressure curve correlates with the moisture content curve, but its rising phase lags significantly behind the soil pressure curve. After water-sediment separation, the seepage zone retains sediment while draining water, which slightly reduces the seepage rate. However, the overall seepage drainage process remains efficient, demonstrating the high feasibility of seepage-based mitigation engineering.