The invention relates to symmetric pyrimidyl iodonium salt and a preparation method thereof. The structural formula of the symmetric pyrimidyl iodonium salt is shown in the description. The preparation method comprises the steps that alcohol reacts with sodium hydride, then the reaction product drips into the reaction product of 5-bromouracil and phosphorus oxychloride, the reaction product reacts with a butyl lithium reagent after reaction and then reacts with simple substance iodine and organic metal halide respectively to obtain a 5-iodine-2,4-dialkoxyl pyrimidine and a metal pyrimidine derivative; the 5-iodine-2,4-dialkoxyl pyrimidine reacts with a nitrogen-fluorine reagent and TMSX-1 to obtain a 2,4-dialkyl pyrimidyl trivalent iodine intermediate and then reacts with TMSX-2 and the metal pyrimidine derivative, then anionic sodium or potassium salt is added to perform extraction, and the symmetric pyrimidyl iodonium salt is obtained. The oxidation method adopted by the preparation method of the symmetric pyrimidyl iodonium salt is acid-free, efficient and moderate. The symmetric pyrimidyl iodonium salt is used for efficient preparation of PET tracer agent <18>F-5-fluorouracil, and the poor problems of poor selectivity, low productivity, difficult separation and the like are avoided.