Objective: To investigate the effect of tofacitinib, a pan-Janus kinase (JAK) inhibitor, on transforming growth factor-beta 1 (TGF-β1)-induced fibroblast to myofibroblast transition (FMT) and to explore its mechanism. To provide a theoretical basis for the clinical treatment of connective tissue disease-related interstitial lung disease (CTD-ILD). Methods: (1) Human fetal lung fibroblast 1 (HFL-1) were cultured in vitro, and 6 groups were established: DMSO blank control group, TGF-β1 induction group, and TGF-β1 with different concentrations of tofacitinib (0.5, 1.0, 2.0, 5.0 μmol/L) drug intervention experimental groups. CCK-8 was used to measure the cell viability, and wound-healing assay was performed to measure cell migration ability. After 48 h of combined treatment, quantitative real-time PCR (RT-PCR) and Western blotting were used to detect the gene and protein expression levels of α-smooth muscle actin (α-SMA), fibronectin (FN), and collagen type Ⅰ (COL1). (2) RT-PCR and enzyme-linked immunosorbnent assay (ELISA) were used to detect the interleukin-6 (IL-6) gene and protein expression changes, respectively. (3) DMSO carrier controls, 1.0 μmol/L and 5.0 μmol/L tofacitinib were added to the cell culture media of different groups for pre-incubation for 30 min, and then TGF-β1 was added to treat for 1 h, 6 h and 24 h. The phosphorylation levels of Smad2/3 and signal transducer and activator of transcription 3 (STAT3) protein were detected by Western blotting. Results: (1) Tofacitinib inhibited the viability and migration ability of HFL-1 cells after TGF-β1 induction. (2) The expression of α-SMA, COL1A1 and FN1 genes of HFL-1 in the TGF-β1-induced groups was significantly up-regulated compared with the blank control group (P < 0.05). Compared with the TGF-β1 induction group, α-SMA expression in the 5.0 μmol/L tofacitinib intervention group was significantly inhi-bited (P < 0.05). Compared with the TGF-β1-induced group, FN1 gene was significantly inhibited in each intervention group at a concentration of 0.5-5.0 μmol/L (P < 0.05). Compared with the TGF-β1-induced group, the COL1A1 gene expression in each intervention group did not change significantly. (3) Western blotting results showed that the protein levels of α-SMA and FN1 in the TGF-β1-induced group were significantly higher than those in the control group (P < 0.05), and there was no significant difference in the expression of COL1A1. Compared with the TGF-β1-induced group, the α-SMA protein level in the intervention groups with different concentrations decreased. And the differences between the TGF-β1-induced group and 2.0 μmol/L or 5.0 μmol/L intervention groups were statistically significant (P < 0.05). Compared with the TGF-β1-induced group, the FN1 protein levels in the intervention groups with different concentrations showed a downward trend, but the difference was not statistically significant. There was no difference in COL1A1 protein expression between the intervention groups compared with the TGF-β1-induced group. (4) After TGF-β1 acted on HFL-1 cells for 48 h, the gene expression of the IL-6 was up-regulated and IL-6 in culture supernatant was increased, the intervention with tofacitinib partly inhibited the TGF-β1-induced IL-6 gene expression and IL-6 in culture supernatant. TGF-β1 induced the increase of Smad2/3 protein phosphorylation in HFL-1 cells for 1 h and 6 h, STAT3 protein phosphorylation increased at 1 h, 6 h and 24 h, the pre-intervention with tofacitinib inhibited the TGF-β1-induced Smad2/3 phosphorylation at 6 h and inhibited TGF-β1-induced STAT3 phosphorylation at 1 h, 6 h and 24 h. Conclusion: Tofacitinib can inhibit the transformation of HFL-1 cells into myofibroblasts induced by TGF-β1, and the mechanism may be through inhibiting the classic Smad2/3 pathway as well as the phosphorylation of STAT3 induced by TGF-β1, thereby protecting the disease progression of pulmonary fibrosis.