Objective: To evaluate and compare the predictive values of preoperative neutrophil to lymphocyte ratio (NLR), postoperative NLR, postoperative/preoperative NLR ratio, preoperative systemic inflammatory response index (SIRI), postoperative SIRI, and postoperative/preoperative SIRI ratio for post-endoscopic retrograde cholangiopancreatography (ERCP)pancreatitis (PEP)in patients with choledocholithiasis. Method: Clinical data including preoperative and postoperative blood routine results, intraoperative ERCP conditions, basic information and medical history of 549 patients with choledocholithiasis undergoing ERCP were collected. The patients were divided into the PEP group(n=43)and the non-PEP group(n=506). The levels of NLR and SIRI before and after ERCP were compared between the two groups, the postoperative to preoperative ratios of NLR and SIRI were calculated, the differences in levels of these indicators between the two groups were compared, and the receiver operating characteristic (ROC) curve was used to analyze the predictive value of each indicator for PEP. Results: Significant differences were observed between the PEP group and the non-PEP group in preoperative NLR, postoperative NLR, preoperative SIRI, postoperative SIRI, postoperative/preoperative NLR ratio and postoperative/preoperative SIRI ratio(all P < 0.05). The areas under the ROC curve(AUC)of postoperative/preoperative NLR ratio and postoperative/preoperative SIRI ratio were both above 0.800, and the postoperative/preoperative SIRI ratio had a higher sensitivity of 95.3% and specificity of 70.2% than other indicators. Conclusions: The postoperative/preoperative NLR ratio and postoperative/preoperative SIRI ratio have reference value for predicting PEP in choledocholithiasis patients undergoing ERCP.