The nitration process as one step in 2-amino-5-chloro-4-methylbenzenesulfonic acid synthesis was improved by optimizing nitration reaction conditions, improving product separation method to eliminate salty scrap acid and cycle use of the concentrated sulfuric acid. The effect of various parameters such as temperature, quality of fuming sulfuric acid, quality of fuming nitric acid, and reaction time was studied for optimization of the nitration reaction condition. In the optimum conditions, the mass ratio of fuming sulfuric acid to sodium 2-chlorotoluene-4-sulfonate and molar ratio of fuming nitric acid to sodium 2-chlorotoluene-4-sulfonate were 4 and 1.05, respectively. The reaction temperature was 5 degrees C, and the time of reaction was 3 h. By applying the optimum reaction conditions, the yield of nitration reaction was increased to 91%, larger than that of the current industrial process whose yield is only 65.7%. The solubilities of sodium 2-chloro-5-nitrotoluene-4-sulfonate in aqueous sulfuric acid solutions were determined by a dynamic method, and the results showed that the lowest sodium 2-chloro-5-nitrotoluene-4-sulfonate solubility was exhibited in 0.7 mass fraction aqueous sulfuric acid solutions. So the product sodium 2-chloro-5-nitrotoluene-4-sulfonate of nitration reaction was separated by filtration from 0.7 mass fraction aqueous sulfuric acid solutions, and the obtained concentrated aqueous sulfuric acid solution was reused.