Objective To investigate the expression level of micro ribonucleic acid-21 (miR-21) in peripheral blood of children with allergic asthma and its correlation with the balance of T helper cell 17 (Th17)/regulatory T cell (Treg). Methods From June 2016 to June 2019, 86 children with allergic asthma in our hospital were selected as the study objects(asthma group), and another 78 healthy children who had physical examination in our hospital in the same period were selected as the control group. The expression level of miR21 in peripheral blood was detected by real-time fluorescence quantitative analysis (qRT-PCR), the Th17/Treg level in peripheral blood was detected by flow cytometry, and the relationship between them was analyzed. Results The levels of eosinophils, miR21, Th17, interleukin (IL)-1β, IL-17, IL-23, IL-6, IL-10, and Th17/Treg value, serum total immunoglobulin E (IGE) level in asthmatic group were significantly higher than those in healthy control group (P＜0.05), and the levels of Treg, IL-12, transforming growth factor β (TGF), lung function related indexes forced expiratory volume in one second (FEV1), forced vital capacity (FVC) and maximum breathing air flow (PEF) were significantly lower than those in the control group (P＜0.05). There was a positive correlation between levels of miR-21 and Th17/Treg in peripheral blood of children with allergic asthma (P＜0.05). The levels of miR21 and Th17/Treg were positively correlated with the levels of IL-1β, IL-17, IL-23, IL-6 and IL10 (P＜0.05), and negatively correlated with the levels of IL-12, TGF, FEV1, FVC and PEF (P＜0.05). Conclusion The expression of miR-21 in peripheral blood of children with allergic asthma is high, which is positively correlated with Th17/Treg level, and miR-21 may promote inflammation and disease development by affecting Th17/Treg balance in children with allergic asthma. 