Objective:To study the correlation between systemic immune inflammation index (SII) and prognosis of patients with hilar cholangiocarcinoma after surgical treatment.Methods:The clinical data of 181 patients with hilar cholangiocarcinoma treated by surgery at the First Affiliated Hospital of Zhengzhou University from January 2012 to December 2016 were retrospectively analyzed. There were 119 men and 62 women, with an average age of 62.4 years. SII was calculated using preoperative routine blood tests. Receiver operating characteristic (ROC) curve was used to obtain the optimal cutoff value of SII. The Kaplan-Meier method was used to draw survival curves and survival rates were compared by log-rank test. The Cox proportional risk model was used to analyze single and multiple factors.Results:The SII area under the ROC curve in predicting postoperative survival was 0.749(95% CI: 0.641-0.858), the optimal threshold was 412.6. Using this threshold, patients were divided into the low SII group (SII≤412.6, n=80) and the high SII group (SII>412.6, n=101). The 1, 3, and 5-year cumulative survival rates of patients in the low SII group were 87.5%, 57.5%, and 26.3%, which were significantly better than those of the high SII group of 71.3%, 39.6%, and 9.9% respectively ( P<0.05). Multivariate analysis showed that SII>412.6 ( HR=2.887, 95% CI: 2.256-7.903, P<0.05) was an independent risk factor for overall survival of patients with hilar cholangiocarcinoma. Conclusion:Preoperative SII had predictive values for postoperative survival of patients with hilar cholangiocarcinoma, SII>412.6 was an independent risk factor for postoperative survival.