<p>Internal erosion in granular soils, governed by threshold-controlled particle migration under hydraulic forces, poses significant risks to geotechnical stability. This study numerically investigates the threshold effects in internal erosion of granular soils using a coupled Discrete Element Method and Dynamic Fluid Mesh (DEM-DFM) approach. The sensitivity of erosion to variations in initial porosity, hydraulic gradients, and fine particle content is systematically analyzed. The numerical model is validated by comparing with experimental data, demonstrating its capability to capture particle migration, clogging, and structural evolution. The results reveal distinct threshold behaviors: (1) A hydraulic gradient threshold of 2.0 governs erosion dynamics, below which erosion rate increases with gradient, and above which clogging dominates. (2) An initial porosity threshold of 0.35 distinguishes compaction-dominated (<i>n</i>₀ &lt; 0.35) and erosion-dominated (<i>n</i>₀ &gt; 0.35) regimes. (3) Fine particle content exhibits a critical threshold of 10%, below which erosion decreases with increasing fines due to localized clogging, and above which fines integrate into the soil skeleton, destabilizing the structure. These threshold effects highlight the nonlinear interplay of factors influencing erosion, providing valuable insights for geotechnical risk assessment and mitigation. The study not only offers theoretical contributions but also introduces a robust numerical framework for analyzing complex soil–fluid interaction problems.</p>