Objective:To investigate the diagnostic value of peripheral blood systemic immune inflammatory in-dex(SII)in histological chorioamnionitis(HCA)and the predictive value of adverse pregnancy outcomes.Meth-ods:A total of 138 pregnant women with suspected HCA were included from January 2021 to June 2022 in Peo-ple's Hospital of Zhengzhou as the research objects.According to the results of placental pathology examination,they were divided into non-HCA group(66 cases)and HCA group(72 cases).The general clinical data,laboratory indicators and adverse outcome indicators of the two groups were compared,and the relationship between the a-bove indicators and HCA was analyzed by multivariate Logistic regression.ROC curve was used to analyze the predictive value of SII for HCA and adverse pregnancy outcomes.Results:①Logistic regression analysis showed that increased neutrophil percentage(Neut％)and SII could increase the risk of HCA(OR＞1,P＜0.05),and in-creased lymphocyte percentage(Lym％)was beneficial for reducing the risk of HCA(OR＜1,P＜0.05).②ROC curve analysis showed that the area under the curve(AUC)of Neut％and Lym％for diagnosing HCA were 0.628 and 0.638,respectively,while the AUC of SII was higher(0.722),and the diagnostic threshold of SII was 974.49 ×109/L.③The rates of postpartum hemorrhage,neonatal infection and premature delivery in high SII group(SII＞974.49 ×103/L)were higher than those in low SII group(SII ≤974.49 ×109/L),and the difference was statistically significant(P＜0.05).The value of SII in predicting postpartum hemorrhage(AUC 0.708)and neonatal infection(AUC 0.848)were higher than that in preterm birth(AUC 0.637).Conclusions:Abnormal Neut％and Lym％levels increase the risk of HCA to some extent,but the diagnostic value of HCA is average.Peripheral blood SII has high diagnostic value in HCA,and has good predictive value for postpartum hemorrhage and neonatal infection,which is expected to be a new indicator for prenatal diagnosis and prognosis prediction of adverse pregnancy outcomes in HCA.