<p>Flow-induced suffusion, a form of internal erosion characterized by the detachment and migration of fine particles through the pore network of a coarser soil matrix under seepage forces, poses a significant challenge in geotechnical engineering due to its detrimental effects on soil structure and stability. Here, we investigate meso-scale pore evolution in well-graded silty sand through laboratory-scale suffusion experiments, integrating high-resolution, non-destructive X-ray computed tomography to capture dynamic structural changes. Our findings reveal a negative linear correlation between areal porosity and the distance from the inlet, alongside an isotropic pore orientation pattern sustained throughout the suffusion process. Notably, we identify, for the first time, a transition in pore size distribution from a power-law regime to a positively skewed pattern, which serves as a meso-scale criterion for suffusion instability. Furthermore, we propose a progressive five-stage suffusion model, encompassing (i) soil saturation, (ii) fluidized fines erosion, (iii) transition of deposited fines to fluidized fines, (iv) deposited fines erosion, and (v) suffusion instability. This model, supported by quantitative formulations of soil volume and mass loss, provides critical insights into the fundamental mechanisms governing suffusion instability. Our framework not only enhances the predictive capability of suffusion onset and progression but also informs the development of targeted mitigation strategies, with significant implications for geotechnical research and engineering practice.</p>