In order to achieve the highly- efficient flame retardancy of polyethylene terephthalate (PET), carbon dots (CDs) with N and Si element were used to modify PET and prepare CDs/PET composites. The molecular structure of CDs was characterized by transmission electron microscopy, X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (FT- IR). The flame retardants of CDs/PET composites were evaluated and analyzed by the limiting oxygen index tester (LOI), vertical combustion meter (UL- 94) and cone calorimeter (CONE). The results show that, when the mass fraction of CDs is 4%, the LOI value of PET is increased from the 21.2% to 29.4%. And during the combustion process, the peak heat release rate of PET drops from 513.2 kW/m² to 433.7 kW/m². The total heat release decreases from 71.9 MJ/m² to 54.0 MJ/m². Both the smoke release rate and total smoke generation all decrease, and the residue amount of PET increases. These demonstrate that, the addition of CDs can reduce the thermal hazard and smoke hazard during PET combustion. In addition, by quantifying the flame retardant mode, it is concluded that the flame retardant effect of CDs on PET is the superposition of three effects: physical barrier, flame inhibition and catalytic charring, and the proportions of the above effects are 36.5%, 24.3% and 10.8%, respectively.