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Objective: To investigate whether kynurenine/aryl hydrocarbon receptor (AHR) affects the maternal-fetal tolerance by involving the differentiation of T helper 17 (Th17)/ regulatory T (Treg) cells, and to provide theoretical basic for new treatment of unexplained abortion. Methods: Flow cytometry (FCM) was used to detect the expression of AHR in peripheral/decidual CD4+ T, Treg, and Th17 cells. The effect of Kyn on the differentiation of peripheral/decidual naïve T-cells under Treg-/Th17-polarizing condition was detected by FCM; enzyme-linked immunosorbent assay was performed to examine the level of Kyn in villus and decidual tissues from normal pregnancy (NP) and unexplained abortion (UA). Student's t-test in the case of two groups or one-way ANOVA in multiple groups was used. Results: AHR expression in CD4+ T-cells was decreased in decidua versus blood in early pregnancy (P < 0.0001). Kyn could promote the differentiation of peripheral and decidual naïve T-cells to Th17 cells under Treg-polarizing conditions (P < 0.01). There was no statistical significance about the concentration of Kyn in decidual or villi tissues between NP and UA, and compared with NP, the expression of AHR in decidual CD4+ T-cells from UA was increased (P < 0.001). Conclusions: Kyn/AHR promotes Th17 and restricts Treg cells' differentiation, which is involved in maintaining the balance of Treg/Th17 cells at the maternal–fetal interface.