Abstract: To optimize the utilization of by-products from Hangbaiju leaves, the steam explosion technique in this study was firstly used to pretreat the leaves, the total phenol and flavonoid contents, phenolic compound composition, antioxidant activity, and in vitro digestion characteristics of Hangbaiju leaves with particle sizes of 20, 40, 60, 80, and 100 mesh were systematically examined. The results showed that steam explosion treatment significantly enhanced the total phenol and flavonoid contents in Hangbaiju leaves. Specifically, the 40 mesh leaves sample showed the highest values with total phenols reaching 32.76±0.09 mg CAE·g⁻¹ and total flavonoids at 43.76±0.98 mg RE·g⁻¹, representing approximately 3-fold and 6-fold increases, respectively, compared to the untreated leaves sample. Notably, 3,4-Di-O-caffeoylquinic acid (1.73~3.32 mg·g⁻¹) was detected in the steam-exploded leaves sample, whereas it was absent in the raw leaves sample. Besides, the concentrations of luteolin-7-O-glucoside, apigenin-7-O-glucoside, apigenin, and acacetin increased significantly (P<0.05), while the content of linarin decreased markedly in the steam-exploded leaves sample (P<0.05). It could be demonstrated that steam explosion significantly improved the release of total phenols and flavonoids during gastrointestinal digestion (P<0.05), along with a notable enhancement in the antioxidant property (P<0.05). These results provide critical technical and theoretical insights for the extraction and application of polyphenols from Hangbaiju leaves, offering substantial implications for the valorization of this resource.