Polyethylene glycol (PEG), a typical representative of the high-efficiency and green synthetic lubricating oils, advances in PEG-based nano-lubricant additives are crucial to pursuing energy conservation and sustainable development. PEG molecules functionalized carbon nitride quantum dots (CNQDs-PEG) with uniform size dis-tribution and well crystal structure was first used as the up-to-date nano-lubricant additives for promoting the lubricating performances of PEG200 lubricating oil. CNQDs-PEG showed that conspicuous tribological perfor-mance in PEG200 lubricating oil, which friction-reduction, wear-resistance, and load-supporting functions can be enhanced by 33.1%, 62.7%, and at least 50.0%, respectively when 0.35 wt% of CNQDs-PEG were added. In addition, CNQDs-PEG has a better performance advantage under high loads and long working hours, exhibiting terrific practical utility. Through systematical analysis of the experimental results, the salient lubricating per-formance of CNQDs-PEG derived from their multiple functions in the different stages of the friction test. Firstly, the CNQDs-PEG/PEG lubrication system depended on the polishing, mending, and rolling effects of CNQDs-PEG to alleviate the friction and wear of the friction surfaces. Then, the lubrication system reacted with the metallic surfaces to establish a complicated and robust tribofilm with an average thickness exceeding 300 nm to protect the frictional interface from direct contact, markedly reducing the friction and wear of the friction pairs.