Instant cereal powders often suffer from poor reconstitution and caking during preparation. This study investigated the effects of twin-screw extrusion and particle-size regulation on the reconstitution properties and structural characteristics of yellow waxy corn flour. Nine extrusion treatments (140–180 °C; 12–18% feed moisture) and four particle-size fractions (40–80 mesh) were screened using caking rate and suspension stability as evaluation indices; no formal process optimization was performed. Three 60-mesh samples produced at 180 °C with different feed moisture levels were then selected as representative treatments for mechanistic characterization. Compared with native flour, the extruded samples showed lower caking, improved suspension stability, and substantially higher water absorption. SEM and DSC analyses demonstrated conversion of intact granules into a porous matrix and a marked decrease in residual gelatinization enthalpy, whereas XRD showed retention of the A-type diffraction pattern. FTIR spectra were broadly similar among samples and did not indicate formation of new major functional groups. These results connect extrusion-induced granular and thermal changes with improved hydration and provide a basis for developing clean-label instant waxy corn products without chemical anti-caking agents.