Soil–water characteristic curve (SWCC) is one of the most important properties of unsaturated soil, and pore size distribution (PSD) curve is commonly used to predict SWCC. In this study, an extended discontinuous deformation analysis (DDA) method is used to reveal the influence of PSD on SWCC from the micromechanism. First, the performance of the extended DDA method is validated by the experimental SWCC. And DDA microsimulation result is in good agreement with the experimental result. Then, the relationship between PSD curve and SWCC is analyzed based on the DDA simulation results. The microsimulation results show that PSD influences SWCC by changing the capillary radius in soil. Air-entry value decreases with the percentage of large pore increase. However, the effect of PSD on SWCC tends to diminish at the residual zone because the micropores do not show much difference between two models. This study extends DDA in unsaturated soil analysis and provides support for PSD-based SWCC prediction from the micromechanism.