Abstract: The interaction of potentials between molecules is important for the molecular interface behavior. Theliquid-vapor interface system composed of water molecules is simulated andstudied with equilibrium molecular dynamics (EMD) simulation method in this paper.The results agree with the known conclusions that there is a marked fall ofpotential between the liquid bulk and the vapor bulk. It is also indicatedthat different types of forces have different effects on the moleculesmoving across the interface. The L-J force acts as a resistance tocondensation while the electrostatic force, which is predominant in theresultant force, acts as motivity. Meanwhile, some typical molecules whichparticipate in the phase-change process, are especially traced. The resultsshow that the process of condensation leads to a drop of thepotential energy of a vapor molecule, which falls into the potential well.Correspondingly, the process of evaporation increases thepotential energy of a liquid molecule, which gets over the potentialbarrier.