Objective:To investigate the predictive value of systemic immune inflammation index (SII) for the overall survival of patients with pancreatic cancer by propensity score matching analysis.Methods:The clinical data of 457 patients with pancreatic cancer admitted to the Affiliated Hospital of Qingdao University from August 2000 to December 2019 were retrospectively analyzed. The age, gender, presence of jaundice, pancreatitis and diabetes, serum CA19-9, total bilirubin level, neutrophil count, platelet count, lymphocyte count in blood, presence of radical surgery, tumor TNM stage, tumor location and the like were recorded. The cut-off value of SII was determined by Youden index. The patients were divided into high and low SII groups accroding to the cut-off value. The propensity score matching was applied to reduce the selection bias of patients. Patients were 1∶2 matched and the caliper value was 0.1. The difference on overall survival between the two groups was compared. The prognostic factors were analyzed by univariate and multivariate Cox regression analysis. Kaplan-Meier was used to draw the overall survival curve to calculate the cumulative survival rate, and the differences between the curves were analyzed by Log-Rank test.Results:The cut-off value of SII was 765. There were statistically significant differences between the high SII group ( n=125) and the low SII group ( n=332) on the presence or absence of pancreatitis, the level of total bilirubin in blood, radical surgery, and TNM stage before the propensity score matching (all P value <0.05). After propensity score matching, there was no statistically significant difference between the high SII group ( n=113) and the low SII group ( n=182) on all the clinical parameters mentioned above except for CA19-9, indicating that the two groups were comparable. Univariate analysis showed that the level of CA19-9, SII, radical surgery and different TNM stage were all related to the overall survival of pancreatic cancer patients. Multivariate analysis showed that high CA19-9 level, high SII, no radical surgery, and worse TNM stage were independent risk factors for short overall survival, and high SII ( HR=1.882, 95% CI 1.446-2.450, P<0.001) was significantly associated with poor prognosis. The overall survival of patients with high SII was obviously shorter than the low SII group ( P<0.001), and the average survival time of patients with high and low SII were 8.86 and 11.38 months, respectively. Conclusions:SII is of great value in evaluating the overall survival of pancreatic cancer patients. Higher SII is associated with shorter overall survival.