Abstract: Pseudo ionic liquids (PILs) refer to ionic compounds composed entirely of cations and anions, whose melting point is generally lower than 150 ℃. PILs are generally included in the category of ionic liquids, and there is no obvious distinction between them. PILs have similar physical and chemical properties to ionic liquids, such as incombustibility, low volatility, low freezing point, good thermal and chemical stability, good molecular structure designability and other unique comprehensive properties. Ionic liquids as pure lubricants are greatly limited by the complex synthesis process and the high cost of raw materials, but they are usually employed as high-performance lubricant additives attributing to their excellent tribological properties and strong boundary film-forming ability. However, there are still few reports on the research of PILs as lubrication additives. The research direction will expand the research depth and application range of PILs. However, PILs have poor solubility in base oil with low polarity because of their large molecular polarity, therefore, as the lubricating additives, the molecular polarity of ionic liquids must be reduced first. The research on the molecular structure design of ionic liquids to reduce the molecular polarity and increase their solubility in base oils provides a strong experimental basis and theoretical guidance for the molecular structure design of PILs. It was found that increasing the length and the number of alkyl chains in ionic liquid molecules and enhancing the overall symmetry of molecular structure can effectively reduce the molecular polarity. Cholesterol, as a kind of steroid compounds, exists widely in organisms and is an important part of all animal life activities. Cholesterol possesses a four-ring molecular structure, including three six-ring and one five-ring, and the special lamellar structure makes it dissolve in the base oil and form a robust and stable adsorption film on the surfaces of friction pairs. Meanwhile, cholesterol has a strong molecular structure designability, so the cholesterol derivatives can be designed and prepared to increase their solubility in base oil and improve their tribological performance as additives. In this work, two kinds of oil soluble ionic liquids of cholesterol-based quaternary phosphorus salts were synthesized and employed as the lubricant additives for PAO-10. The results of static tests and thermogravimetric analysis indicated that the two oil-soluble pseudo ionic liquids had good dispersity in PAO-10 and thermal stability. The results of oscillating reciprocating friction and wear tests manifest that the two kinds of pseudo ionic liquids can significantly improve the tribological performance of base oil for steel/aluminum friction pairs. Scanning electron microscopy (SEM) results indicate that the adhesion wear was the main wear type when the friction pair was lubricated by the blank PAO-10. The wear diameters were decreased dramatically when the mixed oil samples with the two pseudo ionic liquids were used as the lubricants. At this point, the abrasive wear and corrosive wear were the main wear types for the friction pairs. The test results of X-ray photoelectron spectroscopy (XPS) and X-ray energy spectroscopy (EDS) indicated that the active elements (such as P, S, et al.) in the mixed oil samples can have tribochemical reaction with the surface of aluminum matrix in the process of friction. The lubrication mechanism of the two kinds of pseudo ionic liquids can be attributed to the wear resistant compounds such as phosphate and sulfate originated from the tribochemical reaction between the pseudo ionic liquids and the metal matrix.