The invention relates to a method for synthetizing a herbicide-dicamba (2-methoxy-3,6-dichloro-salicylic acid) through catalytic oxidation. The method is characterized in that the dicamba is obtained through oxidation of air, oxygen or ozone by taking 2-substituent 3,6-banair as a raw material and adopting a composite catalyst. The method disclosed by the invention has the advantages that the operation is simple, the raw material is easy to obtain, the cost is low, the catalyst can be recycled, and the method is more environment-friendly and is more suitable for industrial production.