The invention relates to a preparation method of sym-trifluorobenzene and belongs to the field of chemical production processes. The preparation method of sym-trifluorobenzene is characterized by comprising the following steps: (1) mixing potassium fluoride with an aprotic polar solvent to obtain a mixed liquid; (2) adding a reaction primer, nitrobenzene and a phase transfer catalyst into the mixed liquid, wherein the reaction primer is sym-trichlorobenzene or 3,5-difluorobenzene to obtain a reaction system; (3) putting the reaction system obtained in the step (2) into a high pressure kettle, removing air in the kettle and sealing the kettle, and reacting to obtain a product; and (4) rectifying the product which is distilled to obtain the sym-trifluorobenzene. The method provided by the invention is relatively low in energy consumption, few in side reaction, simple in reaction step and easy to operate. The denaturation amount of a solvent is small, and the solvent is easily separated and recovered. Compared with other methods, the preparation method is greener and more environment-friendly.