Background: Acute pancreatitis (AP) is one of the most common gastrointestinal diseases with significant morbidity and mortality, especially in the moderate to severe types. Splanchnic vein thrombosis (SVT) is related to AP but the pathogenesis remains unknown. Aim: To investigate the risk factors of moderately severe acute pancreatitis (MSAP) and severe acute pancreatitis (SAP) complicated by splanchnic venous thrombosis. Methods: The clinical data of 290 patients with MSAP and SAP admitted to Xuanwu Hospital of Capital Medical University between December 2015 and December 2020 were retrospectively analyzed. Patients were divided into 2 groups: 1) with thrombosis and 2) without thrombosis. Sex, age, etiology, severity of AP, platelet (PLT), C-reactive protein (CRP), procalcitonin (PCT), interleukin-6 (IL-6), prothrombin time (PT), activated partial thrombin time (APTT), D-dimer (D-D) levels, type of pancreatic necrosis, proportion and location of pancreatic parenchymal necrosis, location of local complications, CT severity index (CTSI) and modified CT severity index (mCTSI) were recorded. Univariate, logistic multivariate regression analyses and nomogram were used to determine the risk factors for splanchnic venous thrombosis complicated by AP. A receiver operating characteristic (ROC) curve, decision curve, and calibration curve were drawn. Results: Among 290 patients with AP, 71 (24.5%) had SAP, and 219 (75.5%) had MSAP. The median age of all the patients was 49 years; 172 patients (59.3%) had biliary disease, 91 patients (31.4%) had hypertriglyceridemia, 13 patients (4.5%) had alcohol disease, and 14 patients (4.8%) had other diseases. Of the 290 patients, SVT was detected in 35 (12.1%). Univariate analysis showed that the severity of AP, PLT, CRP, PCT, IL-6, PT, D-D, proportion of pancreatic parenchyma necrosis (PPN), necrosis of body-tail, and necrosis involving perihepatic and right subphrenic space, CTSI and mCTSI in the thrombus group were all statistically significant (P < .05). The results of multivariate analysis showed that PLT >= 422 x 10(9)/L, necrosis of body-tail, and necrosis involving perihepatic and right subphrenic space were independent risk factors for AP complicated with splanchnic venous thrombosis. The nomogram incorporating these factors demonstrated good discrimination, calibration, and clinical utility. The area under the curve was as high as 0.845. Conclusion: PLT >= 422 x 10(9)/L, necrosis of body-tail, and necrosis involving perihepatic and right subphrenic space are independent risk factors for SVT. A simple nomogram tool helps in the early, accurate prediction of AP. Early and relevant clinical intervention should be provided.