CO2 foam can control the CO2 mobility and improve the sweep efficiency in reservoirs; however, CO2 foam stabilized solely by surfactants is not stable. Nanoparticles can improve the performance of CO2 foam. The synergistic effect of SiO2 nanoparticles and sodium dodecyl sulfate (SDS) on the CO2 foam stability was studied in this paper. The experimental results show that the synergistic effect requires an SDS/SiO2 concentration ratio of 0.1-0.4. The strength of the effect increases as the SDS/SiO2 concentration ratio increases from 0.1 to 0.17 but then decreases as the ratio further increases from 0.17 to 0.4; thus, a ratio of 0.17 provides the best performance for CO2 foam. The mechanisms of the synergistic effect of SDS and SiO2 include modulating the position of nanoparticle adsorption on the CO2 and liquid interface, improving the interfacial properties of the CO2 foam, and reducing its liquid discharge and coarsening. SiO2 nanoparticles can also improve the CO2 foam performance under high temperatures and pressures. The visual flooding experiment reveals that the addition of SiO2 nanoparticles can improve the stability of CO2 foam in porous media and shows good tolerance of crude oil. SDS/SiO2 foam can increase the pressure differences of the flow in sandpacks after water flooding and improve the oil recoveries markedly. As the SDS/SiO2 concentration ratio increases, the pressure differences and enhanced oil recovery first increase and then decrease. The best CO2 foam flooding performance is achieved at an SDS/SiO2 concentration ratio of 0.17, which is related to the CO2 foam stability. The experimental results provide theoretical support for improving CO2 foam flooding under reservoir conditions.