Abstract: Elastohydrodynamic lubrication state usually appears in the point/line contact parts of high mechanical parts, such as gears, bearings, worm wheels. At the macroscopic level, the point/line contact is the contact between two rough surfaces at the mesoscopic level, and the contact between two convex bodies at the microscopic level. The elastoplastic/plastic deformation occurs when the contact of the micro-convex body on the rough surface occurs under medium/heavy load, so the elastohydrodynamic lubrication contact of the two rough surfaces is transformed into an elastoplastic hydrodynamic lubrication contact. In addition, the contact stiffness of the interface determines the overall stiffness of the mechanical equipment. In order to accurately obtain the normal contact stiffness of the interface under elastic hydrodynamic lubrication and its main influencing factors, based on the idea that the normal contact stiffness of the interface is composed of solid contact stiffness and lubricating oil film stiffness, according to solid elastoplastic theory and fluid dynamics theory, the side contact of the slightly convex body between the interface and part of the hydrodynamic lubrication of the film are analyzed respectively. The contact mechanism of elastoplastic hydrodynamic lubrication on double rough surfaces was revealed from the microscopic point of view, and then the normal contact stiffness model of hydrodynamic lubrication interface was established considering the elastoplastic deformation of the side contact of micro convex body. Through simulation analysis, the influence law of factors (normal load, enrolling speed, surface roughness and lubrication medium characteristics) on the normal contact stiffness of lubrication interface was revealed. The results showed that under the condition of the same speed, roughness and viscosity of lubricating oil, the solid contact stiffness and oil film contact stiffness increased nonlinearly with the increase of normal contact load. Under the same conditions of load, speed and lubricant viscosity, the greater the roughness of the contact surface, the stronger the influence of surface topography on the lubrication regime. The contact stiffness of solids accounted for the main part of the total stiffness of the interface, and the interface was mainly borne by solids. Under the same load, roughness and lubricant viscosity conditions, the contact stiffness of solids gradually decreased with the increase of relative velocity, and the oil film stiffness accounted for the main part of the total stiffness of the interface. Under the same load, roughness and speed conditions, the oil film stiffness basically remained unchanged with the increase of lubricating oil viscosity, and the solid contact stiffness was basically not affected by lubricating oil viscosity. It was of great theoretical and practical significance for improving the dynamic performance and reliability of mechanical equipment to accurately obtain the normal contact stiffness of elastic-plastic hydrodynamic lubrication joint surface per unit area through theoretical modeling.