NOVELTY - Producing 3-cyanobenzoic acid, involves: using 3-methyl-benzonitrile as starting raw material and chlorine by reverse tower-type reactor to react, generating 3-(chloromethyl)benzonitrile; adding 3-(chloromethyl)benzonitrile, phase transfer catalyst comprising vanadyl sulfate and sodium tungstate catalyst, benzyl chloride triethylamine ammonium salt, generating 3-cyanobenzoic acid by hydrogen peroxide oxidation reaction, continuously heating, distilling ethanol, cooling to room temperature, filtering, washing filter cake by water for two times, then re-crystallizing with ethanol and drying.    USE - For producing 3-cyano benzoic acid (claimed) useful as an intermediate for preparing cardiovascular drugs, antibiotics, analgesics and nerve system medicine.    ADVANTAGE - The production method provides total yield of 80% and total content of more than 98%. The product prepared by this method has low cost. The method is suitable for industrial production; and provides small environment pollution. The product prepared by this method has high yield and high purity.    DETAILED DESCRIPTION - Producing 3-cyanobenzoic acid comprises:    (1) in 1000 l of enamel reaction kettle, adding 800 l of 3-methyl-benzonitrile, heating to 25-160 degrees C, using pump to pump the material liquid continuously into the filler tower at 5-40 l/minute flow rate to downward spray, at the same time 5-40 l/minute flow rate uniformly packing tower from the lower part into the 310 kg of chlorine, to form 3-methylbenzonitrile and chlorine backward motion for chlorination reaction of gas phase by sheet type enamel condenser, and a gas-liquid separator, where the liquid returns to the enamel reaction kettle and hydrogen chloride and chlorine gas passes into a tail gas absorbing tank and discharging after the reaction is finished, then the material liquid enamel crystallizing in kettle, cooling to 5 degrees C and filtering to obtain 3-(chloromethyl)benzonitrile;    (2) adding 200 kg 3-(chloromethyl)benzonitrile, 100-800 kg ethanol, 1-3 kg catalyst and 0.2-1 kg benzyl chloride triethylamine ammonium salt phase transfer catalyst into 2000 l of enamel reaction kettle, stirring and raising the temperature, keeping the reaction temperature at 30-500 degrees C and slowly into the reaction kettle adding 150-450 kg 30% hydrogen peroxide, after finishing dripping, retaining temperature for 6 hours, to generate 3-cyanobenzoic acid, continuously heating to 100 degrees C, distilling ethanol; washing two times, cooling to room temperature, filtering, then re-crystallizing filter cake with ethanol, drying to obtain the 3-cyanobenzoic acid white powder. The catalyst is vanadyl sulfate and sodium tungstate in the mass ratio of 1:10-10:1, the use amount of the catalyst is 0.5-1.5 wt.% of 3-(chloromethyl)benzonitrile, the 3-(chloromethyl)benzonitrile and hydrogen peroxide has mol ratio of 1:1-3. The benzyl chloride triethylamine ammonium salt phase transfer catalyst is used in an amount of 3-(chloromethyl)benzonitrile of 0.1-0.5%, and mass ratio of 3-(chloromethyl)benzonitrile and ethanol is 1:0.5-4.