Objective: The neutrophil to lymphocyte ratio (NLR) has been proposed as an inexpensive and widely available marker in many diseases. In this study, we aimed to assess the value of NLR to predict the severity and pancreatic necrosis (PN) in acute biliary pancreatitis (ABP). Methods: 290 patients with ABP were enrolled in this study. The blood samples were collected within 24 hours after AP onset. The optimal cut-off value for NLR to predict severe AP (SAP) and PN was determined by receiver operating characteristic (ROC) curve analysis. ABP model was induced by taurocholic acid sodium in rats. The blood samples and pancreatic tissue samples were collected and we compared NLR in different time points and concentration of taurocholic acid sodium. Results: The area under the ROC curve showed NLR had a predictive performance for SAP (AUC: 0.944, SE: 0.015, 95% CI: 0.915-0.973, p<0.001). The optimal cut-off from ROC curve was 13.38 (sensitivity: 80.0%; specificity: 83.2%). For the prediction of PN, the AUC of NLR was 0.910 (SE: 0.025, 95% CI: 0.861-0.958, p<0.001). The cut-off value of NLR was 9.265 (sensitivity: 97.1%; specificity: 72.7%). In taurocholic acid sodium-induced ABP in rats, the NLR increased significantly and peaked earlier than PN. The NLR was observed to increase along with the increase of the extent of PN. Conclusion: NLR at the initial stage (within 24h after the onset) may serve as a powerful marker for early predicting severity and PN in ABP.