The invention discloses a synthesis method of a medical intermediate 3,5-diiodosalicylic acid. The synthesis method comprises the following steps: reacting an ethanol solution serving as a solvent and elementary iodine serving as an iodizating agent with a salicylic acid to synthetize a 3,5-diiodosalicylic acid crude product; purifying and refining by adopting alkalized acid precipitation method; carrying out segmented heat preservation, crystallization, centrifugation and baking to obtain a product 3,5-diiodosalicylic acid. The method has the advantages that the raw materials and auxiliary materials are easily available, reaction process conditions are mild, the operation is safe and simple, stable in product quality, clean and friendly to environment, the problems in the traditional technology that a reddish yellow IC13 impurity is easily generated by adopting IC1 as the iodizating agent and the content of a single iodine compound is high are solved, the purity is over 99.5%, and the conversion rate can be up to 98.5%.