To enhance oil recovery in ultra-low permeability reservoirs, carbon dots were synthesized via a facile hydrothermal method using citric acid as carbon source and tris(hydroxymethyl)aminomethane (Tris) as nitrogen dopant. A highly active nano oil-displacement system was then constructed by compounding carbon dots with cetyldimethyl betaine. The structural characteristics of carbon dos were characterized by transmission electron microscopy, Fourier-transform infrared spectroscopy, and zeta potential analysis. Reservoir brine stability, interfacial activity, core wettability, oil film stripping capacity, static oil washing efficiency, and core flooding performance were systematically investigated. Experimental results reveal that the carbon dots possess an average particle size of 5.9 nm and abundant carboxyl, amide, and ester functional groups, with a zeta potential of -6.62 mV. Adjusting the pH of the carbon dot product to weak acidity enables excellent stability of the nano displacement system under reservoir aqueous conditions. The composite system of carbon dots and betaine reduces the oil-water interfacial tension to 0.078 mN/m, boosting interfacial activity obviously. The contact angle of formation water on core slices decreases from 102.4° to 22.2°, achieving wettability reversal. Furthermore, the composite system exhibits outstanding oil film stripping and static oil washing performance, with an oil film removal area of 85.86% and a static oil recovery efficiency of 62.68%. Core flooding tests demonstrate that the carbon dot-betaine composite system raises the incremental oil displacement efficiency by 14.47%, better than single carbon dots and single surfactant. This study provides a reference for the research and development of high-performance carbon dot nano oil displacement system.