Sedimentary rock is commonly wet in situ and the existence of water may affect oil adsorption in shale, correspondingly influencing oil fluidity. Molecular dynamics (MD) simulations investigated the water effect on adsorption of oil in montmorillonite (MMT) and illite nanopores in this study. The results show that one obviously adsorbed layer of n-heneicosane (long-chained saturated hydrocarbon) was observed on each surface of the minerals in the hydrocarbon-mineral systems. When water was fully saturated in the mineral pores, a pronounced water adsorption layer accompanied by additional layers that were marginally adsorbed were shown on mineral surfaces. In the hydrocarbon-water-mineral systems, water emerged as the dominant adsorbate, forming a strong wetting layer on both minerals, particularly pronounced in MMT pores. Water would preferentially adsorb to MMT surface and has negative effect on the capture of hydrocarbon. The water effect on hydrocarbon adsorption in illite pore is weaker than that of MMT, especially when the water saturation is low. This study unveils competitive adsorption's nature and its impact on pore phase configurations, shedding light on vital shale system.