The invention discloses a synthesis method of dicamba, which comprises the following steps of: using 2, 5-dichlorophenol as raw materials, conducting sulfonation and bromination reaction to obtain 4-hydroxy-2, 5-dichloro-3-bromobenzenesulfonic acid, treating the 4-hydroxy-2, 5-dichloro-3-bromobenzenesulfonic acid by using alkyl metal complexes of magnesium or lithium, conducting electrophilic addition reaction with CO2 to obtain 5-sulfo-3, 6-diclorsalicylic acid, removing the sulfo of the 5-sulfo-3, 6-diclorsalicylic acid and conducting O-methylation reaction with methyl chloride to obtain the dicamba. The synthesis method of dicamba has the advantages that since the 2, 5-dichlorophenol is used as the raw materials, the raw materials for reaction are easy to obtain, the reaction process is environmental-friendly, the quality of the obtained product is high, the yield is high and the application value is greater.