<p>This study systematically investigates the physical saprolitization mechanism of carbonate rocks in central Yunnan, China. Physical weathering plays a dominant role, at least during the early to middle stages of saprolitization, based on comparative analyses of saprolite samples with varying weathering (powderization) degrees in terms of crystal morphology, chemical composition, and pore structure. Scanning electron microscopy and pore structure analysis reveal that the structural connections between crystal grains are progressively destroyed during the transformation from corestone to saprolite, a process highly consistent with fatigue failure characteristics. Following matrix suction experiments confirming that moisture fluctuations regularly alter suction within rocks, a theoretical analysis was introduced to further explore the triggering mechanism. Inspired by the one-dimensional spherical soil particle model, it is inferred that suction fluctuations accumulate at the grain scale to generate dynamic tensile stresses, providing a plausible source of dynamic loading responsible for fatigue failure. Field evidence further supports this mechanism: Saprolite tends to form near the bedrock–soil interface, as the soil helps maintain a variable-humidity condition around the bedrock. In the same region, saprolites near the surface exhibit a higher degree of powderization than those at depth, and greater powderization also occurs in areas with higher pore connectivity—both consistent with more active moisture migration. This study establishes a closed-loop mechanism linking variable humidity, moisture migration, dynamic loading, and structural degradation, and offers a new perspective on saprolitization beyond the traditional framework of chemical weathering.</p>