4, 5-dicyano dimethyl phthalate was an intermediate of the synthesis of phthalocyanine compounds substituted carboxyl. Under the existence of the solvent methanol, 4, 5-dibromo dimethyl phthalate was synthesized by 4,5-Dibromophthalic acid and thionyl chloride.Then it was with cuprous cyanide to get 4,5-dicyano dimethyl phthalate. By studying the process conditions, we got the optimal reaction conditions of 4,5-Dibromo dimethyl phthalate which was thionyl chloride: 4,5-Dibromophthalic acid (2.5:1.0 , molar ratio) and by enlarging 10 times of the experimental conditions, we got the yield 86.4% which was higher than the reported 66.8%.And the optimal reaction conditions of 4,5-dicyano dimethyl phthalate was that high purity argon, absolutely anhydrous DMF, cuprous cyanide and 4,5-Dibromo dimethyl phthalate whose molar ratio were 4.0:1.0. By enlarging 10 times of the experimental conditions, we got the yield 19.0%.