Cementitious materials are typical quasi-brittle materials. They have been used widely in the construction of civil engineering. In order to avoid unforeseeable brittle failures, steel bars and fibers have been introduced into cementitious system to improve the system’s ductility. However, the ductility of those modified cementitious materials belongs to the pseudo ductility, as cracks will present when the materials deform to a large extent. Therefore, study on cementitious materials’ ductility improvement with other modification methods is worthwhile in view of searching a new way to obtain the true ductility. This work is focused on the utilization of polymer modification to improve the ductility of cementitious composite. A method, in which polymer is interpolated into cementitious system to form a hybrid co-matrix with polymer formation and cement hydration, is used in the present work to modify the cementitious matrix and to improve ductility. Results show that the method is very effective in enhancing the cementitious matrix from quasi brittleness to flexibility. In modification work, epoxy resin (EP), corresponding hardener (HD) and functional silane (FS) were adopted. Functional silane took the role of bridging material connecting polymer and cementitious moiety. Epoxy resin and the selected hardener, which were added into the modified system cooperated forming steric organic structure and enhancing the deformability of the modified system. Based on the mechanical performances, the formula optimization has been carried out. The key parameters, such as P/C ratio, EP-AS/HD ratio, AS/EP-AS ratio, etc. have been studied. The reasonable formula and corresponding curing condition are proposed for the targeted ductility. In experiments, the orthogonal experiment results and the further experiments results reveal that 15%, 4/1 and 0.3 are the optimum values for P/C ratio, EP-AS/HD ratio and AS/EP-AS ratio. Besides, different from traditional curing condition, the steamed curing is recommended. To describe the ...