Hemoglobin glycation index (HGI) is used to quantify the biological variation of hemoglobin A1c (HbA1c). The present study explored the relationship between HGI and coronary artery inflammation in patients with type 2 diabetes mellitus (T2DM). A total of 360 diseased coronary arteries from 185 T2DM patients. Patients and their diseased vessels were categorized into three distinct groups according to the tertiles of HGI: H1 (low group), H2 (medium group), and H3 (high group). Clinical baseline data, perivascular fat attenuation index (FAI) within the proximal 40 mm of the three coronary arteries, quantitative plaque parameters, and the proportion of computed tomography (CT) high-risk plaque features were compared. Moreover, a multivariate logistic regression analysis was used to analyze the risk factors for CT high-risk plaques. The results showed that the perivascular FAI of the left anterior descending artery (LAD) and left circumflex artery (LCX), fibrous, and lipid component volumes, as well as their respective volume ratios, increased with an increase in HGI (P<0.05). The prevalence of CT high-risk plaques also increased with an increase in HGI (P<0.001). HGI (odds ratio (OR) = 1.764, 95% confidence interval (CI) = 1.363-2.284, P<0.001) and LAD-FAI (OR = 1.086, 95% CI = 1.032-1.143, P<0.05) were independent risk factors for CT high-risk plaques. In conclusion, HGI may serve as a potential complementary biomarker for coronary inflammation and plaque vulnerability in patients with T2DM, and warrants further validation in larger prospective studies.