Preparation method 3,4-Dichlorobenzotrifluoride Preparation The p-chlorotoluene was prepared by three steps of photochlorination, fluorination and cyclic chlorination. Chlorine toluene side chain chlorination UV source, the right amount of catalyst, the reaction temperature of 110 ~ 120 °C, time 15 ~ 20h, on the chlorine trichlorotoluene. Its fluorination in the autoclave through anhydrous hydrogen fluoride, pressure 1.6MPa, reaction 4h, and add the appropriate amount of catalyst, the chlorination of benzotrifluoride on the benzene chlorination required catalyst exists, the reaction temperature is 70 °C, the reaction at the end of the material dilute Hydrochloric acid treatment, separation, organic phase washing, drying and other links, get 3,4-dichlorobenzotrifluoride. Preparation of 2-amino-5-hydroxybenzoic acid o-Nitrobenzoic acid and 10% H2SO4 under the action of Pd/C catalyst, reaction at 80～100°C and 957.6Pa for 20h, to obtain 2-amino-5-hydroxybenzene Formic acid. Preparation of 5-[2-chloro-(trifluoromethyl)phenoxy]-2-aminobenzoic acid Condensation of 2-amino-5-hydroxybenzoic acid with 3,4-dichlorobenzotrifluoride, reaction at 135°C 17h, the corresponding diphenyl ether compounds. Preparation of Fomesafen Ether The product of the previous step was passed through phosgene in a toluene solution, reacted at 40-50° C. for 2 h to obtain the corresponding amino acid anhydride, and then reacted with methylsulfonamide (salt) in DMF solvent at 100° C. for 24 h. The corresponding aminolysis product was obtained; finally, concentrated nitric acid and hydrogen peroxide in glacial acetic acid were reacted at 70° C. for 20 h to obtain fomesafen. Preparation Method 2 First, 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid is synthesized and reacted with methanesulfonylthioisocyanate to produce fomesafen. Or with thionyl chloride, into acyl chloride, and then react with methyl sulfonamide fomesafen. This method is more reasonable than the preparation method.