Objective: To measure main degradation product in acetylcysteine granules by HPLC.Methods: HPLC conditions included: C18 column, mobile phase: ammonium sulfate buffer (obtained by diluting ammonium sulfate 2.25 g, sodium heptanesulfonate 1.82 g with water to 450 mL, regulating pH with 7 mol/L HCl solution to 2.0)-methanol solution, detection wavelength: 205 nm, and sampling amount: 10μL.Results: It is verified that degradation product peak of acetylcysteine damaged by light radiation, oxidation, acid, alkali, and high temperature has good separation with main component peak and auxiliary material peak, and main degradation product impurity C peak has good separation with other impurities peaks.The detection limit of acetylcysteine was 0.5 ng.The relative correlation factor of impurity C was 0.9, so the content of impurity C may be accurately measured by self comparison method without adding correction factor.Conclusion: The method has strong specialization, simple operation, high sensitivity, and can be used for measuring content of main degradation product impurity C in acetylcysteine granules.