The invention relates to the technical field of preparing carbon black from junked tires, in particular to a method of preparing the carbon black from the junked tires. The junked tires are smashed and mixed with a desulfurizing agent according to the ratio of 1 to 1, and then the mixture is subjected to ultrasound catalytic reaction; after that, mixed acid is added, and temperature is raised for reaction; magnetic separation, desulfuration, temperature rise ultrasound catalytic reaction and temperature rise and drop treatment are conducted; as a result, a large quantity of impurities in the junked tires are removed, the quality of the carbon black is improved, and particularly the sulfur content in the carbon black is lowered to 0.03% or below; moreover, tire rubber subjected to insufficient splitting remaining in the carbon black is avoided, the quality of the carbon black is improved, application of carbon black products to chemical materials is promoted, and the value added is increased.