AIM: To explore the protective effects of sinomenine (SIN) on oxidative stress and pulmonary fibrosis and its relationship with the Kelch-like ECH-associated protein 1 (Keap1)/nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway. METHODS: MRC-5 cells were treated with hydrogen peroxide (H2O2) to establish the oxidative stress injury model, followed by administration with SIN. Cell viability was detected using the CCK-8 method. The biochemical kits were employed to measure malondialdehyde (MDA) content and superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and catalase (CAT) activities. The protein expression of Keap1 and Nrf2 was examined by western blot. Thirty SD rats were randomly divided into control group, bleomycin A5 (BLM) group and BLM+SIN group, with 10 animals in each group. Bleomycin A5 were intratracheally administered to the rats in BLM group and BLM+SIN group to establish the pulmonary fibrosis model. The rats in control group received the same volume of 9 g/L sodium chloride solution. The second day after model construction, the rats in BLM+SIN group were gavaged with SIN, while the rats in the other two groups were treated with 9 g/L sodium chloride solution. On day 28, all rats were sacrificed. Pulmonary tissue was isolated, and HE and Masson staining was performed to observe the pathological changes. The MDA content and SOD, GSH-Px and CAT activities in pulmonary tissue were evaluated. Western blot was used to assay pulmonary tissues Keap1 and Nrf2 protein expression. RESULTS: When compared with H2O2 group, SIN treatment increased cell viability, decreased MDA content, elevated SOD, GSH-Px and CAT activities, down-regulated Keap1 expression, and promoted nuclear translocation of Nrf2 in MRC-5 cells. In comparison with BLM group, administration of SIN decreased alveolitis and pulmonary fibrosis pathological changes and scores as well as pulmonary tissue MDA content, enhanced pulmonary tissues SOD, GSH-Px and CAT activities, down-regulated pulmonary tissues Keap1 expression, and raised Nrf2 levels in the nucleus. CONCLUSION: SIN alleviates oxidative stress and pulmonary fibrosis possibly by activating the Keap1/Nrf2 signaling pathway.