The aims of this study was to explore the effects of hemoglobinA1C (HbA1c) levels on the flow velocity through arteries of patients treated with percutaneous coronary intervention (PCI) for acute coronary syndrome. A total of 220 acute coronary syndrome patients treated with PCI were selected and divided into two groups based on their HbA1c level on hospital admission: group A (HbA1c < 6.5%) 128 cases; group B (HbA1c ≥ 6.5%) 92 cases. The PCIrelated coronary flow velocity was confirmed using corrected TIMI frame count (CTFC). For the patients in group A, the CTFC for the left anterior descending (LAD) branch was 35.59 ± 6.84 frames, that for the left circumflex (LCX) branch was 26.55 ± 5.92 frames, and that for the right coronary artery (RCA) was 25.79 ± 6.50 frames. For the patients in group A, the CTFC levels for the LAD was 35.59 ± 6.84 frames, that for the LCX was 26.55 ± 5.92 frames, and that for the RCA was 25.79 ± 6.50 frames. The slow flow rate in group A through the LAD was 6.25%, that through the LCX was 8.1%, and that through the RCA was 8.9%; in group B, the slow flow rate through the LAD was 17.2%, that through the LCX was 25%, and that through the RCA was 26.4%. The difference was statistically significant (P < 0.05). HbA1c ≥ 6.5% significantly decreases PCI-related coronary flow velocity and increases the rate of slow flow among acute coronary syndrome patients.