In order to improve the dispersity and stability of nano‐SiO2 aqueous system with high solid content, acrylic acid/allyloxy polyethylene glycol copolymer dispersants with methoxysilicon end groups (KH590‐AAx‐APEGy) were synthesized by photopolymerization. After adding KH590‐AAx‐APEGy into the nano‐SiO2 aqueous system, the viscosity, the curing time, the particle size, and the zeta potential of these systems were respectively measured. The results showed that introducing polyethylene glycol chain into the dispersant was beneficial to better exerting its electrostatic repulsion and steric hindrance effects and further improving the dispersibility of the nano‐SiO2 aqueous system. Moreover, the dispersion stability of SiO2 nanoparticles in water was closely related to the composition, the molecular weight, and the addition amount of KH590‐AAx‐APEGy and the solid content. When the molecular weight of KH590‐AAx‐APEGy was 3200 g mol−1 and its addition amount was 0.1 wt %, the nano‐SiO2 aqueous system with 20 wt % solid content showed the best dispersity and stability. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48617.