The invention relates to the field of compound synthesis, in particular to a synthesis method of an agricultural insecticide thiamethoxam. According to the preparation method of the thiamethoxam provided by the invention, through selecting appropriate reaction raw materials, a new synthetic route is designed, and an intermediate 2-chlorine-5-chloromethyl-thiazole can be prepared at room temperature, so that a cooling device is not needed, the energy is saved, and the reaction yield can be improved.