Abstract: This study aims to investigate the mechanism of wettability alteration of oil-wet calcite surfaces. Using molecular dynamics simulations, the microscopic process of wettability change for single-component and mixed oil molecules on calcite surfaces under waterflooding conditions is examined. The results show that the wettability alteration of calcite surfaces occurs in three stages: the first stage involves the formation of water channels and the disruption of the first adsorption layer of oil molecules; the second stage is characterized by water molecules displacing along the surface via hydrogen bonding interactions with the surface; the third stage reaches the adsorption equilibrium of oil/water/surface. Water molecules can displace alkane molecules but cannot displace mixed oil molecules. The presence of polar molecules in mixed oil leads to stronger hydrogen bonding and shorter bond distances. The distance between H (asphaltene) and O (calcite) is approximately 1.424 Å, while the distance between H (water) and O (calcite) is about 2.273 Å. Polar molecules cannot be displaced from the surface, but water molecules can alter their adsorption configuration from a layered to a droplet-like form, reducing the probability of subsequent oil molecule adsorption. Additionally, the anchoring effect of asphaltene molecules prevents alkane molecules from desorbing. These findings provide a direction for improving recovery efficiency in carbonate oil reservoirs.