With the development of modern industry, rotary lip seals are applied to more complex working conditions. For lip seals with larger or smaller radial force and lip seals with incomplete motion such as robot joint seals, it has been found that the traditional reverse pumping theory has a certain scope of application. Therefore, in this study, a mixed lubrication model based on deterministic surface microdeformation is proposed to calculate the tangent deformation of lip surface asperities without presetting the reverse pumping effect and then assess the effect of reverse pumping behavior. The radial force, contact width, friction torque, leakage rate, static contact pressure, and tangential deformation were either measured or calculated. The results obtained by this study can be used as guidelines for the design and optimization of rotary lip seals.