The invention discloses a synthetic method of histamine dichloride, and belongs to the field of drug synthesis. The synthetic method comprises the following steps: preparing a cyano methylimidazole intermediate through oxidization of TCCA (Trichloroisocyanuric Acid) under an alkaline condition by taking L-histidine as an original raw material, and then preparing medicinal histamine dichloride through catalytic hydrogenation and one-step salifying process. In the histamine dichloride compounded through the synthetic method, the content of related substances is less than 0.5 percent, the content of a single impurity is less than 0.1 percent, and the histamine dichloride accords with medicinal level. A reagent adopted by the technology is cheap and of low-toxicity, the reaction is safe and reliable, an alcohol solvent can be recycled, after-treatment operation is simple and convenient, and the technology is environment-friendly and is beneficial for large-scale industrial production.