The invention provides a purifying method of highly-pure 1,2,3-trimethylbenzene. The method comprises a raw material preprocessing process and an extraction and rectification process, and adopts integrated differential thermal coupling design. The tower top of a preliminary heavy component removal tower T2 supplies heat to the top bottom of a preliminary light component removal tower T1, the tower top of an extraction and rectification tower T3 supplies heat to the tower bottom of the preliminary heavy component removal tower T2, the tower top of a solvent recovery tower T4 supplies heat to the tower bottom of the extraction and rectification tower T3, and steam supplies heat to the solvent recovery tower T4; and the preliminary light component removal tower T1, the preliminary heavy component removal tower T2 and the extraction and rectification tower T3 are respectively provided with reboilers. Sulfolane/dimethyl sulfoxide with the mass ratio of 10-5:1 is adopted as an extractant, and the integrated differential thermal coupling extraction and rectification design is adopted, so the device investment and the operating cost are reduced, and the yield of 1,2,3-trimethylbenzene is improved. A system in the invention allows highly-pure 1,2,3-trimethylbenzene to be effectively extracted from mixed C9 aromatic hydrocarbon solvent oil, the purity of the above product to be not smaller than 99wt% and the yield to be 92-95%.