According to the requirement for dissolution rate of drug granules in the 2010 edition of the State Pharmacopoeia Commission Pharmacopoeia , we established a assay method of the dissolution of erythromycin ethylsuccinate for more effective quality control. According to this new method , we evaluated the erythromycin ethylsuccinate through spectrophotometric method and the sulfuric acid solution (75 → 100) was used as chromogenic reagent. Through the comparison between the method of charge transfer reaction and sulfuric acid colorimetry, we found that sulfuric acid colorimetry method has the merits of better reproducibility and precision, higher sensitivity and more stable after the solution and can be used as the method to determin the dissolution of erythromycin ethylsuccinate.