Abstract: In order to improve the hydration rate of polymer cement-based materials, nano-SiO₂ was added to prepare polymer cement-based composites modified by nano-SiO₂. Combined with multiple microscopic characterization methods including XRD, FTIR, NMR and EDS, this paper systematically investigates microstructure evolution of composites under low-temperature curing conditions, and further discusses the influence mechanism of low temperature on the composites. The results indicate that nano-SiO₂ is still capable of pozzolanic reaction with Ca(OH)₂ at various low temperatures, accelerating cement hydration and improving the compressive strength of polymer cement mortar. The 7-day compressive strength is increased by 27.8%, 25.1% and 24.6%, respectively.Meanwhile, the diffraction peak intensities of unhydrated cement particles and Ca(OH)₂ inside the composite decrease significantly. The stretching vibration peak of Si–O bonds in the gel matrix shifts toward wavenumbers, leading to denser C–S–H gel. The main peak of the T₂ spectrum shifts obviously to the left, large pores transform into small ones, and the pore structure is optimized. The incorporation of nano-SiO₂ can optimize the structure, morphology and distribution of hydration products in the interfacial transition zone (ITZ) between polymer paste and aggregate. Compared with standard curing conditions, lower temperatures result in the deactivation of water in the composite cementitious material and impede the reactivity of nano-SiO₂, which ultimately makes it difficult for the emulsion formed by EVA latex powder to develop a continuous polymer film network. The findings of this study provide a theoretical basis and technical reference for the practical application of nanomaterials in concrete repair and reinforcement systems under low-temperature conditions.