The invention provides a method for detecting particle size of carbon black in PET (polyethylene terephthalate), which comprises the following steps: (1) washing the PET material, drying, crushing into fragments, and carrying out low-temperature plasma spraying treatment on the PET fragment sample; (2) mixing the product obtained in the step (1) with an ionic liquid and dibasic alcohol, reacting at 120-150 DEG C for 0.5-5 hours, filtering the reaction product after the reaction finishes to separate out the residual solid, and washing with deionized water until the pH value is neutral to obtain a first surplus solid; (3) mixing the first surplus solid with an acid solution, carrying out ultrasonic heating at 30-60 DEG C for 0.5-2 hours, and taking out a second surplus solid in the solution; and (4) carrying out ultrasonic dispersion on the second surplus solid to obtain a disperse carbon black suspension, taking out the suspension, drying, taking a photo with a transmission electron microscope, and making statistics on the carbon black particle size. The detection method has the advantages of accurate detection and high operability.