This paper aims to explore the influence of ionic liquid on the tribological performance of used engine oil. After the evaluation for the influence of additives on friction by a four-ball experimental machine, the morphology and surface chemical composition of the wear scars are characterized by SEM, EDS and roughness profiler, and the influence of additives on film formation was studied by a measurement device of point contact light interference oil film thickness. After adding the ionic liquid, at 40 ℃ and 100 ℃, the friction coefficients are lower than those of used oil, which are about 0.095 and 0.085 respectively. Especially at 100 ℃, the friction coefficient (≈0.085) is lower than that of fresh lubrication oil (≈0.09) and the diameter of the wear scar is also reduced. Besides, under the conditions of “light” and “severe” starvation of elastohydrodynamic lubrication (Starved EHL), adding IL to the used engine oil can get better film formation and increase about 20 nm center film thickness. After the addition of IL to the lubricated boundary, the used engine oil has regained good friction and wear performance. During the friction process at high temperature, IL undergoes a complex chemical reaction on the metal surface to form a low shear strength reaction film; In the state of starvation, the oil-based additive IL has a strong polarity and it is easy to form a stable adsorption film on the contact surface, which changes the pressure distribution of the lubricating oil on the contact area and causes the effective viscosity of the lubricating oil in the inlet area to be much greater than its the apparent viscosity, so it improves the film-forming ability of used oil, thereby regaining better lubricating performance.