Objective:To investigate the therapeutic efficacy and underlying mechanisms of the traditional Chinese medicine formula "Hua Xian Fang" (HXF) in the treatment of radiation-induced pulmonary fibrosis (RIPF).Methods:In vivo experiment, 36 male specific pathogen free (SPF)-grade C57BL/6 mice aged 6-8 weeks were randomly divided into the control, irradiation (17 Gy thoracic irradiation), and irradiation+HXF groups (17 Gy thoracic irradiation+HXF). After 16 weeks, lung coefficient, HE staining and Masson staining were used to evaluate the degree of pulmonary inflammation and fibrosis. Immunohistochemistry was performed to measure the expression levels of α-smooth muscle actin (α-SMA) in lung tissues. Quantitative real-time PCR (qPCR) was performed to detect the mRNA expression levels of interferon-γ (IFN-γ). Enzyme linked immunosorbent assay (ELISA) was conducted to determine the levels of IFN-γ in serum and bronchoalveolar lavage fluid (BALF). During in vitro experiment, NIH/3T3 fibroblasts were stimulated with IFN-γ after 6 Gy irradiation, followed by 48 hours of culture. qPCR, immunofluorescence staining, and Western blot were used to assess the expression of α-SMA and collagen Ⅰ at the transcription and protein levels. One way ANOVA was used for multiple group comparisons, and Tukey test was used for inter group multiple comparisons. Results:Compared to the control group, mice in the irradiation group showed significant increases in lung coefficient, Szapiel score, Ashcroft score, and α-SMA expression in lung tissues (all P<0.001). Compared to the irradiation group, the irradiation+HXF group exhibited significant decreases in the above indicators (all P<0.001). qPCR demonstrated that the mRNA expression levels of IFN-γ were significantly higher in the irradiation+HXF group than that in the irradiation group ( P=0.001). ELISA results showed that the levels of IFN-γ in serum and BALF were significantly elevated in the irradiation+HXF group compared to those in the irradiation group ( P=0.032, 0.037). In vitro experiment revealed that after irradiation, the expression levels of α-SMA and collagen Ⅰ mRNA and protein in NIH/3T3 cells were significantly increased, while decreased after IFN-γ stimulation. Conclusion:HXF effectively alleviates RIPF, probably by the upregulation of IFN-γ in blood and tissues and inhibition of fibroblast activation.