According to 45 000 t/a low concentration of sulfuric acid in nitrobenzene production by the Hebei Jiheng Group Corporation Ltd. must be concentrated to return the reuse system, a new technology of synthesis of nitrobenzene by recycling waste acid to absorb NO₂ was developed. The process was based on nitroxyl cation （NO₂ ⁺） instead of nitric acid to make mixed acid, the nitryl cation（NO₂ ⁺）was generated from the aqueous waste acid after nitration reaction absorbing NO₂ gas,and the problem, using the NO₂ gas instead of nitric acid as the raw material of synthesis of nitrobenzene ,was solved, and the production cost was significantly reduced. The reaction conditions were optimized.The absorption of NO₂ the waste acid, gas by the waste acid.The reaction conditions were optimized, at the same time, the temperature, pressure and NO₂ concentration within the waste acid absorber were adjusted. The better technological conditions were the ratio of waste acid and NO₂ was 2.5:100,the pressure in the tower was 0.9 MPa,the temperature was 25 ℃.