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This study aims to explore the role of harmane in ameliorating pulmonary fibrosis in mice by inhibiting the transformation of lung fibroblasts into myofibroblasts. Transforming growth factor-β_1(TGF-β_1, 5 ng·mL~(-1)) was used to treated MRC-5 cells for the modeling of pulmonary fibrosis. The inhibitory rates of different concentrations of harmane on model cells were measured by the CCK-8 method, and the median inhibitory concentration(IC_(50)) was determined. Harmane at IC_(50) and the adjacent concentrations was used for intervention. The cell myofilament immunofluorescence method, scratch experiment, and flow cytometry were adopted to evaluate the inhibitory effects of harmane on the proliferation, differentiation, and migration of model cells. Then, qRT-PCR and immunofluorescence methods were used to analyze the effects of harmonine on the expression of α-smooth muscle actin(α-SMA), type Ⅰ collagen(collagen Ⅰ), and fibronectin at mRNA and protein levels. Fifty SPF-grade male C57BL/6 mice were randomly assigned into a normal control group and a modeling group. A mouse model of pulmonary fibrosis was constructed by intranasal dropping of 5.0 μg·kg~(-1) bleomycin. The modeled mice were randomly allocated into model, nintedanib(30 mg·kg~(-1)) and harmane(30, 45 mg·kg~(-1)) groups, with 10 mice in each group. The mice in harmane and nintedanib groups were administrated with aqueous solution by gavage. After 14 days of continuous administration, the respiratory function of each mouse was measured with a non-invasive whole-body plethysmography system. The pathomorphological characteristics of the lung tissue were observed by hematoxylin-eosin(HE) and Masson staining. The hydroxyproline(HYP) level in the lung tissue was measured by a HYP assay kit. Enzyme-linked immunosorbent assay was employed to determine the content of TGF-β_1, interleukin(IL)-6, and IL-1β in the lung tissue. Western blot was employed to quantify the expression levels of α-SMA and collagen Ⅰ in the lung tissue. The results showed that harmane significantly inhibited the proliferation of MRC-5 cells stimulated with TGF-β_1, with an IC_(50) value of 29.01 μg·mL~(-1). Harmane inhibited the phenotypic differentiation and decreased the scratch migration rate of lung fibroblasts, while significantly promoting the occurrence of apoptosis. In addition, harmane significantly down-regulated the mRNA levels of α-SMA, collagen Ⅰ, and fibronectin. The animal experiment showed that compared with the control group, the model group exhibited significant reductions in body weight and lung function, deposition of collagen fibers, an increase in fibrosis area, elevations in levels of HYP, TGF-β_1, IL-6, and IL-1β, up-regulation in protein levels of α-SMA and collagen Ⅰ, and obvious pulmonary fibrosis. Compared with the model group, the administration groups showed increases in body weight and lung function, reductions in collagen fiber deposition and fibrosis area, declines in levels of HYP, TGF-β_1, IL-6, and IL-1β, and alleviated pulmonary fibrosis. Harmane slows down pulmonary fibrosis in both in vivo and in vitro experiments by inhibiting the transformation of lung fibroblasts and reducing the inflammatory response.