Metal contact seals are widely used in mechanical equipment, and such sealing performance is directly influenced by the interfacial micro-contact state. In this paper, the surfaces with random roughness are simulated by an exponential autocorrelation function and root-mean-square roughness. The model of metal surface micro-contact is established. The influences of the external loads on the interfacial separation, contact area and maximum interfacial stress are investigated by means of Hertz contact theory. Calculations and investigations are carried out for a concrete sealing interface. The variations of the interfacial separation, contact area and interfacial stress with the external applied loads are investigated with the two surfaces of the same and different root-mean-square roughness, respectively. Meanwhile, the concrete variation curves are obtained. Furthermore, the obtained variation curves are analysed in detail.