Objective To investigate the effects of astragaloside II on the interleukin (IL)-21 / signal transducer and activator of transcription 3 (STAT3) pathway and airway inflammation in young asthmatic rats. Methods Young rats were randomly divided into control, model, astragaloside II low-dose, astragaloside II medium-dose, astragaloside II high- dose, and dexamethasone groups with 12 rats in each group. Except for the control group, the groups were challenged with ovalbumin to prepare asthma models and the rats in each group were treated with corresponding drugs. Airway reactivity (Penh value) was measured and the levels of IL-6, IL-21, and transforming growth factoR-β1 (TGF-β1) in serum and bronchoalveolar lavage fluid (BALF) were measured by enzyme-linked immunosorbent assays. HE staining was used to observe pathological changes in lung tissues. Western blot was used to detect protein expression of IL-21, phosphorylated STAT3 (p-STAT3), and STAT3 in lung tissues of the rats. Results Compared with the control group, the structure of bronchial epithelial tissue was disordered, pulmonary epithelial cells were edematous and exfoliated, and inflammatory cells had infiltrated in the model group. After stimulation with acetylcholine (Ach) at various concentrations, the Penh value, total number of leukocytes, numbers of eosinophils, neutrophils, lymphocytes, and macrophages in BALF, the contents of IL-6, IL-21, and TGF-β1 in BALF and serum, the lung tissue inflammation score, and protein expression of IL-21 and p- STAT3 in lung tissues were increased significantly (P< 0.05). Compared with the model group, the lung tissue structure of young rats in astragaloside II treatment groups and dexamethasone group was gradually complete, after stimulation with Ach at various concentrations, the Penh value, total number of leukocytes, numbers of eosinophils, neutrophils, lymphocytes, and macrophages in BALF, contents of IL-6, IL-21, and TGF-β1 in BALF and serum, the lung tissue inflammation score, and protein expression of IL-21 and p-STAT3 in lung tissues were decreased significantly (P< 0.05), which were dependent on the dose. Conclusions Astragaloside II alleviates airway inflammation in young asthmatic rats, repairs lung tissue and the bronchial structure, and improves asthmatic symptoms by inhibiting the IL-21 / STAT3 pathway.