The invention discloses a method for preparing 2-chloro-5-fluoropyridine-3-benzoic acid represented by a formula I, which comprises the step of reacting a compound II and a formate in a solvent in the presence of a palladium catalyst to obtain the compound I. The preparation method of the invention has the advantages that: the formate is used as a reducer, so the equipment corrosion by formic acid adopted in the traditional method is reduced, the production and operation environments are improved and an acid bonding agent is not required; the synthesis process is simple, the cost is low, the yield is as high as 80 to 90 percent, the product purity is high, and industrial production is suitable to be carried out; and the preferred ionic liquid used is environmentally-friendly and simple inpost treatment and can be repeatedly used.