Abstract: Objective To detect the expression of helper T cells 17 (Th17), regulatory T cells (Treg), Th17/Treg and interleukin-17(IL-17) in peripheral blood of patients with unexplained recurrent spontaneous abortion (URSA), and explore the role of imbalance of immune factors in the pathogenesis of URSA. Methods From January 2015 to October 2017, 162 URSA patients in Xiangya Hospital Central South University were selected as group URSA, 90 cases of normal early pregnancy as early pregnancy group, and 90 cases of normal unpregnant as unpregnant group. Th17 cells, Treg cells and Th17/Treg in peripheral blood were measured by flow cytometry, and the level of serum IL-17 was measured by ELISA. Pearson correlation analysis was adopted to analyze the correlation between Th17/Treg and IL-17 in patients with URSA. Results The levels of Th17, Th17/Treg and IL-17 in group URSA were significantly higher than those in the unpregnant and early pregnancy groupsTh17(%):(2.85±0.73) vs (1.31±0.24)and (1.42±0.28); Th17/Treg:(0.79±0.12) vs (0.28±0.04) and (0.31±0.06); IL-17(pg/ml):(38.75±6.13) vs (18.52±4.05)and (20.18±4.16), all P< 0.01. The content of Treg cells in group URSA was significantly lower than that of unpregnant and early pregnancy groups(3.75%±0.84%) vs (4.92%±1.18%) and (4.83%±1.06%), P< 0.01, respectively. And there was no significant difference in levels of Th17, Th17/Treg and IL-17 between early pregnancy group and unpregnant group (all P> 0.05). The correlation analysis showed that Th17/Treg was positively correlated with IL-17 in URSA patients (r=0.805, P< 0.01). Conclusion The high expression of Th17/Treg and IL-17 is related to the occurrence of URSA, and the imbalance of immune factors may be one of the reasons for the mediation of URSA.