In-situ Polymerized Modified Cementitious Composites (iPMCC), where monomers polymerize within the cement matrix leading to a more pronounced increase in flexural strength, have recently garnered significant attention from researchers. By introducing calcium acetate into the iPMCC system, the polymerization reaction and cement hydration process are promoted, resulting in the formation of a highly entangled polymer network that strengthens the interfacial bonding between hydration products and the polymer. The results demonstrate that calcium acetate-modified iPMCC exhibits enhanced flexural strength and bend toughness, without a significant reduction in compressive strength. This study provides a viable strategy for developing high-performance polymer-modified cement-based materials and underscores the critical role of the synergistic effect between in-situ monomer polymerization and calcium ions in fabricating advanced cement composites.