Medicine intermediates 2, 4-difluorobenzoic acid synthesis method, comprises the following steps : the reaction vessel was added 3-4 mol 1, 3-dichloro-2-propanol solution, 2 mol 2, 4-difluoro-6-bromo-isopropylbenzene solution, 600-800 mL aqueous solution, controlled stirring speed at 110 -130rpm, raised the temperature of the solution to 40-50 癈, added 5-6mol boron trifluoride ether solution, keep for 90-110min, after the end of the reaction, under vacuum concentration, added 300m1 1-pentanol solution, filtered, adjusted the pH of the filtrate to 4-5, after the crystals are precipitated, filtered and recrystallized in the 4-methyl-2-pentanone solution, and dehydrated with the dehydrating agent to obtain the product 2, 4-difluorobenzoic acid.