The invention relates to a method for preparing dicamba and belongs to the technical field of herbicide dicamba. The method comprises that benzene as a raw material is subjected to directional chlorination and catalysis, the product is purified to form 1, 4-dichlorobenzene, the 1, 4-dichlorobenzene is halogenated and hydrolyzed in the presence of a catalyst to form 2, 5-dichlorophenol, 3, 6-dichlorosalicylic acid is prepared from 2, 5-dichlorophenol and the 3, 6-dichlorosalicylic acid undergoes methylation, saponification and acidification reactions to produce dicamba. Through process and parameter optimization, the method has simple processes, realizes a low cost, a high yield and high selectivity, observably reduces a waste water amount and improves a device use rate.