Trimesitinic acid, a special carboxylic acid, is usually synthesized from 1,3,5-trimethylbenzene by oxidation of KMnO_4 in based aqueous solution, but lower yield is a problem. This paper is trying to explore the synthetic conditions systematically in the aspects of temperature, pH value, solvent, phase-transfer catalysts, etc. ,respectively. The experiment obtains some practical conclusions and a better condition which could give a good product yield. The affecting factors have been fully disscused.