Abstract: Owing to the addition of nano SiO₂ can promote the hydration rate, improve the mechanical properties and interfacial transition zone (ITZ) and refine the pore structure of polymer cement-based composites, the microscopic mechanism of influence of nano SiO₂ on the early properties of polymer cement-based composites was revealed by means of XRD, SEM, EDS, microhardness (MH) and mercury intrusion porosimetry (MIP) experiments. The results show that when nano SiO₂ content is 2wt%, the mechanical properties of the polymer cement-based composites are the best. The compressive strength is 57.5 MPa and 67.3 MPa in 3 d and 7 d age, respectively, which is 12.7% and 13.9% higher than that of the polymer cement-based composites with polymer simply. The addition of nano SiO₂ changes the hydration products and microstructure of the polymer cement-based composites. As for ITZ, the thickness of ITZ between polymer cement hardened paste and aggregate decreases, its morphology becomes denser, the calcium-silicate ratio in ITZ declines and the microhardness in ITZ increases by nano SiO₂ incorporation. Since nano SiO₂ can further fill finer pores of the polymer cement-based composites, there is higher proportion of gel pores and the mean pore diameter tends to be smaller so that the pore structure of the polymer cement-based composites is greatly optimized by nano SiO₂ addition.