<p>This study investigated the microstructural evolution of phyllite particles under confined compression to support their engineering application as fill materials. Confined compression tests combined with micro-computed tomography (μCT) scanning were conducted to characterize the internal mesostructure of phyllite specimens under different stress states. The CT images were batch processed using Avizo software to reconstruct two- and three-dimensional models of particles and pores. Particle breakage, pore distribution, pore equivalent diameter, and particle sphericity under low stress were quantitatively analyzed, whereas particle-shape evolution under high stress was evaluated qualitatively based on CT observations. The results showed that particle breakage occurred extensively during confined compression and initiated preferentially at the specimen ends and edges. Increasing stress promoted particle displacement, edge grinding, splitting, and fine-fragment migration, thereby reducing particle size and enhancing pore filling. Meanwhile, the pore network evolved into a denser structure dominated by small pores. The average two-dimensional porosity decreased from 0.407 at 1 MPa to 0.127 at 9 MPa, corresponding to a reduction of approximately 68.8%, while the minimum porosity decreased from 0.273 to 0.055. The average particle volume was 1.12 × 10⁴ mm³ for the 16–31.5 mm fraction and 5.60 × 10² mm³ for the &lt; 16 mm fraction, indicating that low stress mainly induced particle rearrangement and edge grinding, whereas high stress promoted intensive fragmentation and pore filling.</p>