On the basis of investigations of the theory of surface phenomena, the interface layer model of physical interface is proposed in this paper. The concepts of respective interfacial tension and surface forces in the internal and external regions of two phases are proposed. Calculation equations for these parameters are deduced. Surface forces are discussed on the basis of the theory of intermolecular interactions. The contribution of the inductive forces to the surface energy is taken into account in the theoretical calculation. Results of the calculation show that the theoretical model proposed in this paper is reasonable.