Objective To study the portal venous systemic thrombosis (PVST) in early acute pancreatitis (AP) and its correlations with the classification and severity of AP. Methods A total of 396 patients with AP were admitted to the affiliated hospital of north sichuan medical college from January 2013 to May 2017 and underwent MRI in the early stage of AP. PVST was evaluated on the T1WI, T2WI fat-suppression, and dynamic-enhancement sequences. Evaluating the MR imaging, AP was graded as mild, moderate, and severe AP based on the MR severity index (MRSI) and was also classified into interstitial edematous AP and necrotizing AP. According to the New Revised Classification of AP 2012, AP in the clinic setting was graded as mild, moederately severeand severe AP. χ2 test or Fisher exact test calculated the differences of the prevalence of PVST in different severity and classification of AP, Mann-Whitney U test calculated the difference of hospitalization time between patients with PVST and those without PVST. Results Among the 396 patients with AP, PVST was detected in 30 patients (7.5％,30/396), it formed most frequently in splenic vein(73.3％, 22/30), followed by portal (30.0％, 9/30) and superior mesenteric(16.7％, 5/30) veins. According to MRSI, there were 205, 177, and 14 patients with mild, moderate, and severe AP, respectively;among mild, moderate, and severe AP, there were 2, 21, and 7 patients with PVST, respectively (χ2=41.455, P<0.01), there were also statistical differences in the prevalence of portal and splenic vein thrombosis (P<0.05), but there was no statistical difference in the prevalence of superior mesenteric vein thrombosis (P>0.05). Three hundred and eleven patients had interstitial edematous AP and 65 patients had necrotizing AP, among which there were 11 and 19 patients with PVST(χ2=48.447,P<0.01), the prevalence of portal, splenic and superior mesenteric vein thrombosis in necrotizing AP were all higher than that in interstitial edematous AP (P<0.05). Based on the New Revised Classification of AP 2012, there were 194, 184 and 18 patients with mild, moderately severe, and severe AP, respectively; among mild, moderately severe, and severe AP, there were 0, 25, and 5 patients with PVST, respectively (χ2=42.130, P<0.01), there was no statistical differences in the prevalence of portal, splenic and superior mesenteric vein thrombosis (P>0.05). Patients with PVST and those without PVST in the early AP, the hospitalization time [median (interquartile range)] were 18 (13 to 22) days and 13 (10 to 19) days (Z=-2.913, P=0.004). Conclusion PVST in early AP presented more frequently with the increase in severity of AP based on both the MRSI and Newly Revised Classification of AP 2012, along with longer duration ofhospitalization.