The effect of the thermal composite fluid remaining in the reservoirs on the rheological properties of the plugging agent particle solution is still unclear at the late thermal composite development stage of the heavy oil reservoirs. Therefore， it limits the application of particle solutions in this stage. For this， based on the rheological theoretical model and viscosity-concentration formula，a viscosity-concentration relationship that can accurately describe the SiO₂ + polymer dispersion system was preferably selected by combining the viscosity-concentration change of particle solutions. The effects of different factors on the rheological properties of the SiO₂ + polymer dispersion system and the mechanism were investigated by using a rheometer，a Zeta potential analyzer，and a scanning electron microscope. The results show that the viscosity-concentration relationship of the SiO₂ + polymer dispersion system is in accordance with the Krieger and Dougherty viscosity-concentration models. The viscosity of the SiO₂ + polymer dispersion system decreases with increasing temperature or mineralization. The viscosity of the SiO₂ + polymer dispersion system in-creases with increasing particle concentration or pH. The difference in viscosity between SiO₂ + polymer dispersion systems with anionic and nonionic surfactants is not significant，while the viscosity of SiO₂ + polymer dispersion system with cationic surfactants is significantly greater than that of the remaining two solutions. The viscosity of the particle solutions with different types of surfactants decreases as the pH decreases. Therefore，it is important to select a suitable particle solution to provide a theoretical basis for the application of SiO₂ + polymer dispersion system in the late thermal composite development stage of thick oil reservoirs.