<p>The initiation of debris flows is a macroscopic evolution process resulting from particle–particle and particle–fluid interactions, yet the underlying microscopic mechanisms remain difficult to elucidate through traditional laboratory model tests. To address this challenge, this study focuses on a typical source accumulation body at stake K1562 along the Oytak-Bulunkou section of the China-Pakistan Highway. Based on field investigations and laboratory tests, the particle shape and gradation characteristics of the source material were obtained. The CFD-DEM coupling method was employed to conduct numerical simulations and macro–micro analyses of the debris flow initiation mechanism, with particular emphasis on the influence of particle shape on the initiation process and evolution patterns. The results indicate that: Polyhedral particles exhibit higher mechanical coordination numbers and redundancy indices compared to spherical particles. As the particle aspect ratio increases, the interparticle contact number, mechanical coordination number, and redundancy index all increase, thereby enhancing force transmission capacity and improving the soil’s resistance to external loads. Increasing the aspect ratio significantly strengthens the interlocking effect among non-spherical particles. Angular particles demonstrate greater resistance to runoff than perfect spheres. The initiation-evolution mechanisms vary substantially among source materials with different particle shapes. With increasing aspect ratios, slope blocking becomes more pronounced-polyhedral particles with an aspect ratio of 2.0 exhibited blocking-burst phenomena during failure. Changes in particle shape alter the slope failure process, manifesting as an evolutionary sequence from sliding and mass movement to relative stability.</p>