To investigate the early-stage Th17/Treg ratio in the peripheral blood of acute ischemic stroke (AIS) patients and its relationship with 90-day prognosis. Methods: We assigned 64 patients with AIS and 40 healthy controls to the AIS group and control group, respectively. The levels of Th17 and Treg cells and the ratio of Th17/Treg in the peripheral blood of all participants were detected by flow cytometry. Patients were divided into the good prognosis group (n=37) and poor prognosis group (n=27) according to the mRS score after 90 days. The clinical data of the two groups were compared. Spearman analysis was used to assess the relationship between Th17/Treg and severity of condition. Multivariate logistic regression analysis was used to examine the influencing factors of a poor 90-day prognosis. The receiver operating characteristic curve (ROC) was used to evaluate the predictive effect of the Th17/Treg ratio at time of admission. Results: The peripheral blood Th17 level was increased, Treg level decreased, and Th17/Treg ratio significantly increased in AIS group patients compared to control group patients (all P<0.05). The poor prognosis group showed significantly higher peripheral blood Th17/Treg, IL-6 level, infarct volume, and NIHSS score compared to the good prognosis group (all P< 0.01). IL-6, NIHSS, and infarct volume were positively correlated with Th17/Treg (P<0.001). Infarct volume and Th17/Treg were independent risk factors for a poor 90-day prognosis. The area under the ROC curve for Th17/ Treg in predicting AIS prognosis was 0.898 (95%CI: 0.828～0.971, P=0.000). The optimal cutoff value of Th17/ Treg was 3.775 with a sensitivity of 66.7% and a specificity of 97.3%. Conclusion: The early-stage Th17/Treg ratio in peripheral blood increased in patients with AIS and offered a certain predictive value for 90-day poor prognosis.