4,5-Dibromophthalic acid was an intermediate of the synthesis of phthalocyanine compounds substituted carboxyl. Under the existence of the solvent methylene chloride, 4,5-Dibromo-o-xylene was synthesized by O-xylene and bromide through electrophilic substitution reaction.Then it was by the oxidation with potassium permanganate to get 4,5-Dibromophthalic acid. By studying the process conditions, we got the optimal reaction conditions of 4,5-Dibromo-o-xylene which was Br2 volume concentration 40%，O-xylene volume concentration 40%，Br2:I2: O-xylene（3.0:0.15:1.0 , molar ratio）and by enlarging 10 times of the experimental conditions, we got the yield 79.0% .And the optimal reaction conditions of 4,5-Dibromophthalic acid was that potassium permanganate, phase transfer catalyst and 4,5-Dibromo-o-xylene whose molar ratio were 6.0:0.04:1.0 , reaction time 10 h ,the temperature 100 ℃. By enlarging 10 times of the experimental conditions, we got the yield 85.5%.At last, the yield we got was higher than the reported 71.0% and the 77.0%.