The interfacial tension (IFT) is an important factor for the hydrocarbon flow in a porous reservoir. Methane, one main accompanied gas of hydrocarbon, has a crucial influence on the IFT. However, the methane effect is still unclear. In this work, the effects of the methane content, temperature, and pressure on the water-oil interface were investigated, employing molecular dynamics simulations. The interfacial density profiles were given and indicated that the methane molecules accumulate at the interface, leading to a decreasing IFT. As the methane mole fraction increases, the interfacial roughness and interfacial thickness increase and the induced deeper molecular penetrations and stronger miscibility initiate a decrease of the IFT. Further, an enhancing fluid diffusivity at the interface is observed, which accounts for the strengthening interfacial mobility. On the other hand, our calculations indicate that the IFT decreases with the rising temperature while increases with the strengthening pressure. Our study provides an in-depth understanding of the interfacial behavior in the ternary phase system and has some promise for the exploitation of shale oil.