Carboxyl-functionalized graphene lubricating oil and a preparing method thereof are disclosed. The lubricating oil includes, by mass, 0.01-4 wt% of carboxyl-functionalized graphene, 0.1-10 wt% of a graphene matrix dispersant, 75-85 wt% of base oil and 15-25 wt% of a lubricating oil additive. The method includes subjecting graphene oxide to carboxyl functionalization modification and reduction to obtain the functionalized graphene; mixing the carboxyl-functionalized graphene, the graphene matrix dispersant and the base oil; subjecting the mixture to heating, stirring, ultrasonic oscillation, stirring and ultrasonic oscillation; then adding the lubricating oil additive into the mixture; and fully stirring the mixture to obtain the lubricating oil. A graphene dispersing issue is solved in theaspect of the mechanism. Compared with traditional lubricating oil, the carboxyl-functionalized graphene lubricating oil has a friction coefficient 10-15% reduced than that of traditional lubricatingoil under same test working conditions, and has a small wear scar diameter. The preparing method is simple in process, low in cost, low in production efficiency, and prone to industrial production, and has good economic benefit and social benefit.