Objective To investigate the relationship of serum autoantibodies against oxidized low-density lipoprotein IgM (ox-LDL-Ab IgM) with coronary lesions and clinical prognosis in patients with ST-segment elevation acute myocardial infarction (STEMI). Methods From August 2008 to May 2009, 95 patients with STEMI undergoing elective coronary angiography (CAG) and emergent percutaneous coronary intervention (PCI) were enrolled in the study. All patients were divided into single-, double- and triple-vessel lesion groups according to the numbers of vessels with significant stenosis. The ox-LDL-Ab IgM, maleic dialdehyde (MDA), serum lipids, body mass index (BMI), Gensini’s score, and cardiovascular events during hospitalization were recorded and calculated. The relationship of serum level of ox-LDL-Ab IgM with coronary lesions and short-term prognosis was analyzed. Results The serum level of ox-LDL-Ab IgM was reduced with the increase in stenostic vessel numbers in STEMI patients (P＜0.05), but had no correlation with Gensini’s score (P＞0.05). The serum level was negatively correlated with serum MDA (r＝-0.319, P＜0.05), but no obvious correlation was found with serum lipids and blood glucose (P＞0.05). There was negative correlation between the serum level of ox-LDL-Ab IgM and cardiovascular events during hospitalization (r＝-0.708, P＜0.05). Logistic linear regression analysis showed that ox-LDL-Ab IgM was a protective factor for patients with STEMI (OR＝-0.588, P＜0.05). Conclusion The serum level of ox-LDL-Ab IgM reflects the severity of atherosclerosis in patients with STEMI. ox-LDL-Ab IgM may reduce modification of LDL to ox-LDL through regulating oxidative stress, and then attenuates the coronary atherosclerosis area. It may play a protective role in the short-term prognosis in STEMI patients.