The invention discloses a synthetic process of herbicide tembotrione. The synthetic process comprises the following steps: step 1) adding methyl 2-chloro-3-methyl-4-methylsulfonylbenzoate, a solvent,a catalyst and hydrobromic acid at the first, then dropwise adding hydrogen peroxide, washing with water, concentrating and recrystallizing after reaction to obtain bromine methyl 2-chloro-3-bromomethyl-4-methylsulfonylbenzoate; step 2) enabling the bromide, an alkali 1, the catalyst, the solvent and 2,2,2-trifluoroethanol to react, filtering, washing with water and concentrating after reaction; adding an alkali 2 and water to react, acidifying, filtering, washing and drying to obtain an etherate 2-chloro-3-(2,2,2-trifluoroethoxy)methyl-4-methanesulfonylbenzoic acid; and step 3) removing the solvent after reacting the etherate, the catalyst, thionyl chloride and the solvent; adding 1,3-cyclohexanedione and the solvent, dropwise adding triethylamine; adding acetone cyanohydrin after reaction, washing with water, layering, removing the solvent from an oil layer, adding the solvent for recrystallization, filtering and drying to obtain a beige solid, namely tembotrione. The synthetic process provided by the invention increases the yield of an intermediate, is more environmentally-friendly and safer, and reduces production costs.