The invention discloses a preparation method of 3, 4, 5-trifluoritrobenzene, and relates to the field of organic synthesis. The preparation method particularly includes steps of adding DMSO, 3, 5-dichloro-4-fluoronitrobenzene in an anhydrous reaction bottle, opening the bottle and stirring; heating to 70-90 DEG C under the reduced pressure; stirring and then adding KF, TBAF; depressurizing and dewatering; after a distilling head is free from water bead, setting temperature at 120 DEG C and ultrasonic wave of 15-30 KHZ, and reacting; after finishing the reaction, inserting an ultrasonic wave reactor probe to a reaction system; tracking the reaction progress through GC; after finishing the reaction, cooling to 70-75 DEG C, and filtering; rectifying filter liquor and separating DMO from the product. The preparation method provided by the invention is simple in operation, fast in reaction, low in energy consumption, and high in product yield.