In this study, the effects of wheat bran dietary fiber (WBDF) particle size on the physicochemical properties and structural conformation of myofibrillar protein (MP) were investigated. The results showed that ultrafine comminution effectively reduced the particle size of WBDF from 138.00 μm to 17.53 μm, maintaining a high extraction purity of 90%. Compositional analysis revealed D-xylose and L-arabinose as the dominant monosaccharides (>91%). While the intrinsic hydration capacities of isolated WBDF decreased with smaller particle sizes, the incorporation of micronized WBDF significantly enhanced both the water-holding capacity and oil-holding capacity of the MP composite system. Furthermore, the micronized WBDF significantly stabilized the MP through a novel "hydrophilic shielding" mechanism. Notably, the addition of finer WBDF increased turbidity from 0.20 to 0.47 but reduced surface hydrophobicity (H0) from 1239.10 to 875.90, decreasing by 29.3%. This reduction, coupled with intrinsic fluorescence quenching and a red shift in maximum emission, indicates the formation of a hydration shell around MP molecules by the arabinoxylan-rich fibers, effectively masking hydrophobic domains rather than exposing them. Furthermore, micronized WBDF facilitated the transformation of MP secondary structures toward a more ordered state, characterized by increased α-helix content and enhanced thermal stability. These findings reveal that micronized WBDF can function as a filler and a functional stabilizing agent, enhancing protein structural stability through hydrophilic modification and offering a sustainable strategy for valorizing wheat by-products in meat gel systems.