Objective To investigate the preventive therapeutic effects of CPD1, a novel phosphodiesterase 5 inhibitor, on lung pathological phenotype and interstitial fibrosis of lung in pulmonary fibrosis model rats caused by bleomycin (BLM). Methods Rats were randomly divided into a sham surgery group (n=10), a model group (n=14), a CPD1 treatment group (n=13), and a nintedanib positive control drug treatment group (n=13). Pulmonary fibrosis model was constructed by slowly instilling BLM (3 mg/kg) into the left bronchus in the model group and two treatment groups. Two hours after BLM infusion, the rats were treated with CPD1 (20 mg·kg–1·d–1), or positive control drug nintedanib (50 mg·kg–1·d–1) by intragastric administration, respectively, for 2 weeks. To observe the effect of CPD1 treatment on pathological structural damage, collagen deposition, and the expression of fibronectin (Fn), α-smooth muscle actin (α-SMA), Collagen Ⅰ, and E-cadherin (E-Cad) in the affected lung tissues of unilateral pulmonary fibrosis rats. Moreover, to further observed the effects of CPD1 intervention on the expression of transforming growth factor β1 (TGF-β1) and Smad3 in the cell model of human alveolar basal epithelial A549 cells. Results Compared with the control group, the lung tissue structure was seriously damaged in the BLM group, and with expansion of the alveolar space, collapse of the alveolar lumen. Significant widening of the alveolar septum and thickening of the alveolar wall were observed in the BLM group. There was a marked increase in collagen deposition in the thickened walls of the BLM group. Moreover, the expressions of Fn, α-SMA, Collagen Ⅰ, TGF-β and Smad3 were increased, while the expression of E-Cad significantly decreased in the BLM group (all P<0.05). Compared with the BLM group, the lung tissue damage was significantly improved in the CPD1 group rats. Furthermore, CPD1 inhibit the expression of Fn, α-SMA, Collagen Ⅰ, TGF-β and Smad3, and upregulate the expression of E-Cad (all P<0.05). Conclusions Prophylactic treatment with phosphodiesterase 5 inhibitor CPD1 strongly attenuates BLM-induced pulmonary fibrosis by inhibiting the lung injury and inflammation response via targeting TGF-β/Smad pathway, reducing the deposition of extracellular matrix.