Objective To investigate the correlation between serum proprotein convertase subtilisin/kexin 9 (PCSK9) level and the severity of angiographic coronary artery disease (CAD) in elderly patients with acute myocardial infarction (AMI) complicated with type 2 diabetes mellitus (T2DM). Methods The patients over the same period were divided into three groups:the elderly patients with AMI and T2DM (observation group, n=46), the elderly non diabetic patients with AMI (positive control group, n=34) and patients with chronic stable coronary heart disease (negative control group, n=33). The serum PCSK9 level was detected by enzyme-linked immunosorbent (ELISA) in three groups. The correlation between serum PCSK9 level and low density lipoprotein cholesterol (LDL-C), high density lipoprotein cholesterol (HDL-C), triglycerides (TG), total cholesterol (TC), glycosylated hemoglobin (HbA1c), high sensitive C reactive protein (Hs-CRP) and Gensini score were analyzed. Results The levels of PCSK9, HbA1c, Hs-CRP and Gensini score showed a gradually increased tendency in negative control group, positive control group and observation group. Multiple comparison differences between groups were statistically significant (P<0.05). The serum PCSK9 level was positively correlated with Hs-CRP and Gensini score in observation group (P<0.05), and showed no correlation with HbA1c, TG, TC, LDL-C and HDL-C (P>0.05). Conclusion The serum level of PCSK9 is positive correlated with the severity of CAD, which can be used to determine the severity of coronary artery lesions in elderly patients with AMI complicated with T2DM.