The aging of lubricants is a primary factor contributing to rolling bearing failures. The rolling stability test simulated the aging behavior of grease under shear and high-temperature circumstances. An investigation was conducted on how mechanical-thermal aging impacts the chemical and physical structures of polyurea grease, as well as how changes in thickener microstructure affect rheological and tribological properties. The results show that under shear and high temperature, structural changes in urea lead to a transformation of the thickener morphologies from the growth of entangled ribbon fiber to a large-diameter rod structure, with a significant decrease in the degree of entanglement. When a certain proportion of fibrous and rod structures coexist, the grease has good oil separation capacity, the structural stabilities of grease become weak, and the deformation resistance decreases. During this phase, the polyurea grease has the best tribological performance. •Mechanical-thermal aging mechanism of polyurea grease elucidated by quantitative infrared spectroscopy.•During aging, the diameter of the thickener structure becomes larger and the entanglement degree decreases.•The best oil separation capacity is achieved when fibrous and rod-like structures coexist.•Proper mechanical thermal aging improves the tribological properties of greases.