Abstract: The crack resistance of repair layers is critical for the durability of rehabilitated concrete structures. Although Engineered Cementitious Composites (ECC) exhibit high ductility, their performance stability is often limited by meso-scale matrix heterogeneity. To address stress concentration and crack control challenges, this study optimizes particle gradation using the Fuller distribution theory to enhance matrix packing density and uniformity. Computer simulation, pore structure analysis, and microhardness tests were employed to evaluate the improvement and its effect on cracking behavior. Results show that the optimized matrix reduces the pore distribution variation coefficient by 52.8% and refines the characteristic pore diameter from 22,331 nm to 3,868 nm. The microhardness variation coefficient decreases from 117.5% to 45.1%, effectively mitigating stress concentration. Consequently, crack width is stabilized below 0.1 mm with an average spacing under 1.5 mm, fully realizing the material’s ductile potential. Additionally, using 70% fly ash as cement replacement significantly reduces carbon emissions, improving environmental sustainability. This work provides an effective material design strategy for enhancing the toughness and durability of concrete repair layers.