Objective This study aims to explore the effect of miR-140-5p on the function of breast cancer cells under hypoxia. Methods The breast cancer cell line MDA-MB-231 was routinely cultured and treated with hypoxia. The expression level of miRNA-140-5p in MDA-MB-231 cells under hypoxia and normoxia was detected by qt PCR. The protein expression level of miRNA-140-5p in MDA-MB-231 cells under hypoxia and normoxia was detected by WB experiment. The cells were transfected with miRNA-140-5p mimics. The cells were divided into groups, and the cells in each group were proliferated Migration and invasion ability testing; Finally, detect the expression of NOX4 in each group of cells and evaluate the regulatory relationship between miR-140-5p and NOX4.Results The expression of miRNA-140-5p decreased in the hypoxic state of MDA-MB-231 cells (P＜0.05), while the expression of miRNA-140-5p mimics increased after transfection. miRNA-140-5p mimics could enhance the proliferation, migration, and invasion ability of cells in hypoxic environments; miRNA-140-5p mimics could also enhance the expression of NOX4 in MDA-MB-231.Conclusion The expression of miRNA-140-5p in breast cancer cell MDA-MB-231 under hypoxia is reduced, and the expression of miRNA-140-5p is increased after transfection with Mimic, and it has an impact on the migration, invasion and proliferation of MDA-MB-231 under hypoxia, indicating that the impact of miRNA-140-5p on the function of breast cancer cell MDA-MB-231 under hypoxia may lead to the occurrence and development of breast cancer. And this mechanism may be formed by inducing changes in NOX4 in MDA-MB-231