Abstract: Objective To explore the expression level of HLA-E, HLA-F and HLA-G in placenta tissue and its relationship with illness state and pregnancy outcome in patients with pregnancy-induced hypertension. Methods Total 81 pregnant women with preeclampsia in our hospital between January, 2016 and December, 2016 were enrolled and divided into mild preeclampsia group (32 cases) and severe preeclampsia (49 cases); 30 healthy pregnant women were selected as controls. The umbilical cord blood was extracted to detect serum HLA-Ⅰ level with ELISA kits, and the placenta tissue was extracted to test the expression levels of HLA-E, HLA-F and HLA-G by immunohistochemical method. In addition, the pregnancy outcome was record, and the difference between women with and without adverse pregnancy outcomes was compared. Results The serum HLA-Ⅰ level was successively decreased in accordance with the order of the control group, mild preeclampsia group and severe preeclampsia group, and the difference between each two groups had statistical significance (P<0.05). The positive rate and expression levels of placenta HLA-E, HLA-F and HLA-G were both successively decreased in accordance with the order of the control group, mild preeclampsia group and severe preeclampsia group, and the difference between each two groups had statistical significance (P<0.05). The rate of adverse pregnancy outcome in preeclampsia group was higher than that in the control group, the difference had statistical significance (P<0.05). The serum HLA-Ⅰ level and placenta HLA-E, HLA-F and HLA-G levels in the preeclampsia group was lower than those in the control group, the difference had statistical significance (P<0.05). Conclusion The decrease of HLA-E, HLA-F and HLA-G expression may be closely related to hypertension and adverse pregnancy outcomes during pregnancy.