The critical attribute was analyzed in clavulanate potassium tablet of amoxicillin according to the principle QbD. By investigation of the drug impurity profile, the cycle-closed dimer and penicilloic acid of amoxicillin were considered to be the critical impurities, and the sources and the degradation pathways of these two impurities were discussed. The research confirmed that crystal form was the critical attribute of drug substance. The drying process in the tablet granulation was regarded as the critical process parameter. The tablet formulation was also another factor in the impurity generation. This study provides a new idea for the evaluation of drug quality.