Objective To explore a green extraction process for flavonoids from Daidai's physiological fallen fruits and to analyze the composition, microstructure, and antioxidant activity of the extracts.Methods Flavonoids were extracted from Daidai's physiological fallen fruits using an ethanol-water solution combined with ultrasonic-assisted extraction. The extraction conditions were optimized through single-factor experiments. The flavonoid composition of the extract was determined by high-performance liquid chromatography (HPLC), its microstructure was observed using scanning electron microscopy (SEM), and in vitro antioxidant activity was evaluated by DPPH radical scavenging, ABTS radical scavenging, and FRAP assays.Results The optimal extraction conditions were: 75% ethanol, 50 min extraction time, 55 ℃, a 1∶45 (g/mL) solid-liquid ratio, and two extraction cycles, achieving a yield of (98.22±0.46)%. The main flavonoid components were naringin, naringenin, narirutin, and nobiletin, with naringin showing the highest content. SEM images showed the flavonoid extract exhibited a fragmented, glass-like morphology of various sizes. In vitro antioxidant assays demonstrated that the extract had significantly higher DPPH and ABTS radical scavenging activities and FRAP reducing power than the raw fallen fruit material.Conclusion The ethanol-water ultrasonic-assisted extraction method efficiently recovers flavonoids from Daidai's physiological fallen fruits. The extract is rich in multiple flavonoid components and exhibits excellent antioxidant activity.