In the pursuit of environmentally friendly tribological systems, water-based lubricants have garnered significant attention, yet issues related to aqueous lubricant still hinder their broader application. In this study, tannic acid (TA) was assembled onto the graphitic carbon nitride (g-C 3N 4) nanosheets via melamine as a cross-linking agent to form TA-g-C 3N 4, followed by the grafting of chito-oligosaccharide (COS, a polysaccharide with amine groups) via a Michael addition reaction, thereby yielding chitosan-oligosaccharide modified g-C 3N 4 nanosheets (COS-g-C 3N 4). The grafting COS imparts COS-g-C 3N 4 with good dispersion stability in water. Compared with pure water, the average friction coefficient of COS-g-C 3N 4 is decreased to 0.20 from 0.57. The improved lubricating behavior arises from the synergistic effect between a friction-induced chemical protective film and the hydration layer. Furthermore, the incorporating COS-g-C 3N 4 into water-based lubricants effectively inhibits bacterial growth during extended storage periods, offering a prospective avenue for the development of multifunctional water-based lubricants.