The invention relates to a preparation method of herbicide dicamba. The preparation method includes: (1) liquid potassium hydroxide and 2, 5-dichlorophenol are reacted according to molar ratio of 0.95:1-1:1 to obtain 2, 5-dichlorophenol potassium; (2) at the presence of anhydrous potassium carbonate and catalyst, the 2, 5-dichlorophenol potassium obtained in the step (1) is reacted with CO2 to generate 3, 6-dichlorosalicylic acid, pressure of CO2 is controlled to be 4-6MPa, reaction temperature ranges from 100 DEG C to 160 DEG C, and molar ratio of anhydrous potassium carbonate and 2, 5-dichlorophenol potassium is 1-2:1; and (3) in alkaline condition, at the temperature of 70-100 DEG C, the 3, 6-dichlorosalicylic acid obtained in the step (2) and chloromethane are reacted under the action of the catalyst through a tube fixed-bed reactor according to molar ratio 1:1-3.5, saponification and acidification are performed to obtain 3, 6-dichloro-2-methoxysalicylic acid, namely the dicamba. The process is high in reaction yield, simple in reaction condition, good in product quality, small in three wastes and low in energy consumption.