Objective To explore the mechanism of placental neutrophils and IL-17 in adverse pregnancy outcomes of pregnant mice infected with Toxoplasma gondii. Methods C57BL/6 pregnant mice were randomly divided into uninfected group and infected group. Each mouse in the infected group was intraperitoneally injected with 0.2 ml suspension of T. gondii RH strain (about 400 tachyzoites per mouse) on day 8 of gestation. The mice in the uninfected group were intraperitoneally injected with the same amount of sterilized PBS on day 8 of gestation. On day 14 of gestation, the two groups of mice were euthanized by cervical vertebra dislocation to inspect by dissection and record the pregnancy outcome. The placental tissue were sampled to prepare paraffin sections for observing histological changes and infiltration of polymorphonuclear grannulocyes by hematein-eosin (HE) staining to calculate the infiltration index. Additional placental tissues were collected to prepare single-cell suspension, with which the percentages of neutrophils in the placental immune cells were detected by flow cytometry. Immunohistochemical staining was used to detect the distribution and localization of T. gondii and IL-17 expression level (gray value). Spearman correlation analysis was used to analyze the relationship between the IL-17 expression and neutrophil infiltration index. Data between two groups were compared using independent sample Student’s t-test. Results Compared with the uninfected group, the infected group showed significant bleeding and congestion in fetal mice and placenta, fetal dysplasia. The abortion rate in the infected group was 82.35% (42/51), which was higher than that in the uninfected group 4.08% (2/49) (t = 7.683, P < 0.01). HE staining showed that the placental tissue of pregnant mice infected with T. gondii had enhanced hepatocellular contour and was accompanied by a large number of polymorphonuclear granulocytic infiltration. The infiltration index was 14.500 ± 0.965, which was significantly higher than that of the uninfected group (3.917 ± 0.633) (t = 9.168, P < 0.01). Flow cytometryanalysis showed that the percentage of neutrophils in placental cells in the infected group was (13.700 ± 1.790)%, which was significantly higher than that in the uninfected group (4.783 ± 0.723)% (t = 5.107, P < 0.01). Immunohistochemical staining analysis showed that a large number of T. gondii could be detected in the placental tissue and mainly concentrated in neutrophils. The IL-17 intensity in the placenta tissue of the infected pregnant mice (17.510 ± 1.372) was significantly higher than that of uninfected pregnant mice (8.178 ± 1.293) (t = 4.951, P < 0.05). Correlation analysis showed that the expression level of IL-17 in the placenta was positively correlated with the degree of neutrophil infiltration (R² = 0.652, P < 0.01). Conclusion T. gondii infection can lead to a significant infiltration of large number of neutrophils and a high level of IL-17 expression in the placenta tisseus of pregnant mice. The elevated expression level of IL-17 may relate to the recruitment of neutrophils at the maternal-fetal interface.