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Objective: To optimize the extraction amount of flavonoid from petals, leaves, stamens and pistils of Camellia japonica, respectively, and compare the antioxidant activity and analyze the chemical constituents of the extracts. Method: The total flavonoids extraction amount was used as evaluation index, and single-factor experiments and orthogonal tests were used to optimize the ultrasonic-assisted enzymatic extraction process of flavonoids. The antioxidant capacity of optimized flavonoids extract was evaluated by DPPH and ABTS+ free radical scavenging and reducing power experiment. And the chemical constituents of extracts from each part were qualitatively analyzed by ultra-performance-liquid-chromatography quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS/MS). Results: In the optimum extraction condition, the total flavonoids extraction amount form each part of Camellia japonica were in the order as follows: Leaves (63.14 RE mg/g)>stamens and pistils (58.77 RE mg/g)>petals (20.26 RE mg/g). Meanwhile, the results of antioxidant activity determination showed that the IC50 values of DPPH and ABTS+ free radical scavenging assay of extracts from petals, leaves, stamens and pistils were 7.28, 2.51, 1.15 mg/mL, and 2.85, 0.95, 0.59 mg/mL, respectively. And the reducing power value of extracts from petals, leaves, stamens and pistils were 63.25, 214.11, 475.90 Trolox mg/g. Therefore, the antioxidant activities of the extracts were in the order of stamens and pistils>leaves>petals. At last, a total of 29 flavonoid-related compounds were identified from the three parts by UPLC-QTOF-MS/MS, and the distribution pattern of B-type procyanidin dimer, epicatechin, quercetin-3-O-galactoside and some other flavonoid compounds in the three parts was consistent with that of total flavonoids extraction amount. Conclusion: Ultrasound-assisted enzymatic extraction effectively improved the extraction amount of flavonoids, and the leaves and stamens and pistils parts of Camellia japonica possessed higher flavonoid contents and stronger antioxidant activity.