Background: Hypoxia is the hallmark of the tumor microenvironment (TME) and plays a critical role during the progress of tumor development. A variety of microRNAs (miRNAs) transmitted by tumor-derived exosomes were involved in intercellular communication. We aimed to elucidate the precise mechanism by which tumor cell-derived exosomes promote lung cancer development by affecting macrophage polarization under hypoxic conditions. Methods: CD163 signal in tumor tissue from lung cancer patients was detected by immunohistochemical (IHC). The M2 polarization-related markers were assessed by flow cytometry and western blot. Exosomes were isolated from normoxic and hypoxic lung cancer cell culture and characterized by transmission electron microscope (TEM), dynamic light scattering (DLS), and western blot. RNA sequencing was performed to show the abnormally expressed miRNAs in exosomes from normoxic and hypoxic lung cancer cell culture. In addition, CCK-8 and clone formation assays were used to assess cell proliferation. Dual luciferase reporter assay was used to evaluate the relationship between miR-21 and IRF1. For in vivo experiment, the male nude mice were injected with H1299 cells with exosomes and miR-21 mimic treatment. Results: Firstly, we found a strong CD163 signal in tumor tissue from lung cancer patients by IHC. Subsequently, we co-cultured lung cancer cell line H1299 with M0 macrophage THP-1 and found that H1299 in a hypoxic environment promoted THP-1 M2 polarization. PKH67 fluorescence staining experiments confirmed that exosomes of H1299 origin were able to enter THP-1 and induced M2 polarization. RNA sequencing of exosomes showed that miR-21 level was significantly higher in the hypoxic culture group compared to the normoxic group. Subsequent cellular assays showed that miR-21 inhibited the expression of IRF1 by targeting it. In addition, the overexpression of IRF1 reversed the role of miR-21 on macrophage M2 polarization. Finally, we have confirmed through animal experiments that either hypoxic environment or high miR-21 level promoted tumor progression. Conclusions: High miR-21 level in hypoxic environments promoted macrophage M2 polarization and induced lung cancer progression through targeting IRF1.Keywords: Exosome; Hypoxia; IRF1; Lung cancer; Macrophage M2 polarization; miR-21.