Abstract: In order to investigate the influence of the number of cationic carbon chains on the tribological properties of ionic liquids, three protone-type ionic liquids with the same anion and chain length, N₁P(N8HHHDEHP), N₂P(N88HHDEHP) and N₃P(N888HDEHP), were selected as additives to the polyA olefinic base oil (PAO10), and the differences in lubrication properties of the liquid were investigated by experiments. The UMT friction and wear testing machine was used to investigate its anti-friction and anti-wear performance in terms of addition concentration and friction frequency. The wear amount was measured by confocal microscope, the wear marks were observed by SEM scanning electron microscope, and the chemical elements were analyzed by EDS spectrometer and X-ray photoelectron spectroscopy (XPS). The film forming properties of three proton-type ionic liquids were investigated by using an optical interference lubricating oil film test rig. The results showed that the addition of ionic liquid could reduce the friction coefficient and wear amount, and the elemental analysis showed that the active elements N and P react with the metal substrate to form tribo-film. Under the same conditions, the lower the number of cationic carbon chains, the better the anti-friction and anti-wear properties and film forming properties of ionic liquids. As the number of cationic carbon chains in ionic liquid decreased, the steric hindrance effect between anions and cations decreased, which was conducive to the formation of network hydrogen bonds, the interaction between anions and cations was enhanced, and a more compact and ordered adsorption film can be formed on the surface of the substrate, thus affecting the quality of the tribo-film, so it has better tribological properties. The adsorption film characterization test confirmed that the ionic liquid could form an adsorption layer on the friction surface, which played a surface modification role, thus reducing the degree of oil depletion, and the less the number of carbon chains, the thicker the adsorption layer. Therefore, the study of the influence of cationic structure on tribological properties was helpful for the molecular structure design of ionic liquid additives with excellent tribological properties and film forming properties.