Abstract： Objective To observe the incidence of severe acute pancreatitis (SAP) in obese acute pancreatitis (AP) patients with medical treatment, and evaluate the impact of obesity in AP progression.Methods A multicenter prospective controlled study was conducted. APACHE Ⅱ scoring system was used to evaluate the severity of AP. Results 161 patients with mild AP(MAP) were enrolled, according to the cut-off point of 25 kg/m2, these patient were divided into obese group (79 patients) and non-obese group (82patients). The levels of CRP, hypertriacylglycerolemia, complication rate, incidence of SAP and mortality were observed under the circumstance of identical medical treatment. The levels of CRP in obese group and non-obese group were (117±109 ) mg/L and (35±36 ) mg/L(P<0.01). The number of obese patients with hypertriacylglycerolemia was two times as many as that in non-obese patients, but there was no significantly difference. There was no local complication in both groups, but the incidence of systematic complication in obese patients (20.3%) was significantly higher than that in non-obese group (6.1%, P<0.01). 16patients (20.3%) in obese group progressed into SAP, which was significantly higher than that in non-obese group (5 patients, 6.1%, P<0.01). One patient(1.3%) died in obese group, but no one died in non-obese group. In MAP patients with APACHE Ⅱ 4～7 points, the incidence of SAP (43.3%) in obese group was significantly higher than that in non-obese group (18.5%, P<0.05). Conclusions Obese MAP patients with APACHE Ⅱ 4～7 points were prone to develop into SAP. More aggressive interventions are needed.