Objective：To explore the value of prostaglandin E2 （PGE2） level in peritoneal fluid in the diagnosis of spontaneous bac－terial peritonitis（SBP） and prediction of in-hospital mortality. Methods：Patients with decompensated cirrhosis and ascites admitted to The First Affiliated Hospital of Chongqing Medical University from August 2016 to February 2018 were enrolled. Patients’ demograph－ic information and laboratory test results were collected. Peritoneal fluid samples were collected，and the PGE2 levels were measured by enzyme-linked immunosorbent assay. The sensitivity and specificity of PGE2 level in the diagnosis of SBP were analyzed by the receiver operating characteristic（ROC） curve. Univariate and multivariate logistic regression analyses were used to evaluate the pre－dictive value of PGE2 level for in-hospital mortality. Results：A total of 193 patients with liver cirrhosis and ascites were enrolled，and 30（15.5%） of them were diagnosed with SBP. The SBP group had a significantly lower PGE2 level than the non-SBP group[32.77（26.05-39.68） pg/mL vs. 49.72（37.35-55.14） pg/mL]. The ROC analysis showed that in the diagnosis of SBP，PGE2 level in peri－toneal fluid had an area under the ROC curve of 0.80，and had a sensitivity of 93.3% and a specificity of 58.3% at a cut-off value of 40.30 pg/mL. Peritoneal fluid PGE2≤32.88 pg/mL was a predictor of in-hospital mortality in patients with decompensated cirrhosis （OR=7.690，95%CI=2.357-25.088，P=0.001）. Conclusion：PGE2 level in peritoneal fluid can be used as a valuable biomarker for the diagnosis of SBP and the prediction of in-hospital mor－tality in patients with decompensated cirrhosis.