Objective To investigate the inhibitory effect of astragaloside II (AS-II) on the proliferation of pulmonary artery smooth muscle cells (PASMCs) induced by hypoxia and its relevant mechanism. Methods Rat primary PASMCs were divided into normoxia group, hypoxia group, hypoxia combined with 20, 40, 80 μmol/L AS-II treated groups, hypoxia combined with nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) inhibitor VAS2870 treated group, and then cultured either in normoxic (210 mL/L O2) or hypoxic (20 mL/L O2) condition for 24 hours. The proliferation of PASMCs was detected by CCK-8 assay. The level of intracellular reactive oxygen species (ROS) was detected by DCFH-DA staining. Protein kinase B (AKT), phospho-AKT (p-AKT), mammalian target of rapamycin (mTOR), phospho-mTOR (p-mTOR), proliferating cell nuclear antigen (PCNA), NOX1 and NOX4 protein expression were assessed by Western blotting. Results In the hypoxia group, the proliferation of PASMCs, level of intracellular ROS, protein expression of PCNA, p-AKT, p-mTOR, NOX1 and NOX4 increased significantly compared with those in the normoxia group. However, AS-II treatment inhibited hypoxia-induced PASMCs proliferation, decreased the level of intracellular ROS, and suppressed protein expression of PCNA, p-AKT, p-mTOR, NOX1 and NOX4. Moreover, VAS2870 treatment lead to similar changes. Conclusion AS-II can inhibit the proliferation of PASMCs induced by hypoxia, which may be associated with the blocking of NOX/ROS/AKT/mTOR signaling pathway.