Abstract: In order to investigate the thermal elastohydrodynamic lubrication characteristics of the x-gears system under dynamic load, considering the influence of modification coefficient and time-varying meshing stiffness, based on dynamic theory of gear system, a six-degree-of-freedom tribo-dynamics model was established. Thermal elastohydrodynamic lubrication characteristics of x-gears systems under vibration and static load were analyzed. The research showed that the positive transmission gear system had the best lubrication effect compared with other transmission types, and a thick lubricating oil film can be formed between the teeth, the friction coefficient between the teeth and the oil film flash temperature was the smallest. As the modification coefficient increased, the lubrication condition was continuously improved, and the thermal gluing load capacity was enhanced; the increment of the modification coefficient increases the meshing stiffness of the gear system, but reduces the stiffness of the oil film.