<p>One of the crucial factors influencing the mechanical properties of loess is its particle size distribution (PSD). The PSD affects the soil’s density, porosity and structural stability, which in turn impacts its compressibility and shear strength. To investigate the precise mechanisms, a series of numerical samples with varying ranges of PSD and percentages of clay particles were generated to carry out triaxial and confined compression tests using the discrete element method. By integrating capillary and van der Waals forces, the model captures microstructural mechanisms that prior PSD-based studies overlooked. The results indicate wider PSD or higher clay content induces a “high coordination but weak force chain” state, where increased fine particles elevate coordination numbers but reduce average contact force, weakening load-bearing capacity. The coupling of capillary and van der Waals forces shifts the critical state line (CSL) to lower void ratios with a shallower slope, distinct from capillary-only models. Clay particle agglomeration and spatial heterogeneity significantly affect mechanical responses, explaining discrepancies between simulations and experiments. This study provides a comprehensive analysis of particle size distribution affecting the mechanical behaviours of loess, providing a foundation for future research and innovation.</p>