Reservoir wettability is very important to predict the structure and residual capture capacity, potential CO₂ leakage accidents and improve the risk assessment and sequestration safety during CO₂ geological sequestration. The wetting behavior of fluid on reservoir rock surface is actually an interface phenomenon caused by the interaction of different fluid molecules on rock surface. At present, it is difficult for laboratory experiments to observe the microscopic interface phenomena at the molecular scale under reservoir conditions, while molecular dynamics simulations can characterize the microscopic changes in wetting behavior and analyze the interaction between the fluid and the rock surface. In this study, molecular dynamics simulations are used to study the effects of different CO₂ densities and different salinities on the wetting behavior of formation water on the sandstone surface during CO₂ geological sequestration. The results show that both salinity and CO₂ density have significant effects on the wettability of quartz surfaces. However, the increase in salinity can only weaken the wettability, while the increase in CO₂ density can transform the wettability of the quartz surface. Both CO₂ molecules and H₂O molecules form two adsorption layers at 3.5 Å and 6.9 Å away from the quartz surface. CO₂ molecules will replace the positions originally occupied by water molecules on the quartz surface, reducing the contact area between water droplets and the quartz surface. The CO₂ density mainly affects the interface behavior between the components in the system, while the NaCl concentration mainly affects the interaction between H₂O molecules and ions in the system. The research results can further improve the understanding of the wettability of fluid on the pore surface of reservoir rocks during CO₂ geological sequestration.