Objective: Glycemic variability (GV) still remains unclear whether acute glycemic excursion has the important prognostic significance in ST-segment elevation myocardial infarction (STEMI) patients undergoing p-PCI. So our aim is to assess the prognostic value of GV in STEMI patients undergoing p-PCI. Methods: We studied 237 STEMI patients undergoing p-PCI, whose clinical and laboratory data were collected. We used a continuous glucose monitoring system (CGMS) to measure the fluctuations of blood glucose. Participants were grouped into diabetes group and non-diabetes group, and grouped into tertiles of mean amplitude of glycemic excursions (MAGE). The major adverse cardiac events (MACE) of patients was documented during in-hospital and 30-day follow-up. The relationship of MAGE and the incidence of MACE were analyzed. Results: Data from 237 subjects were incorporated into the statistical analysis, a higher MAGE level was associated with the higher peak CK-MB values (r = 0.374, P < 0.01), and the higher peak cTnI values (r = 0.410, P < 0.01). The rate of composite MACE by MAGE tertiles (< 2.37 mmol/l, 2.37-3.65 mmol/l and > 3.65 mmol/l) was 7.5% vs. 14.1% vs. 22.8%, respectively (P = 0.025); STEMI patients with a higher MAGE level had a significantly higher non-IRA revascularization compared with those with lower MAGE levels (32% vs. 15% vs. 21%, P = 0.037). Moreover, diabetic patients with higher MAGE level had significantly higher incidence of composite MACE and non-IRA revascularization, non-diabetic subjects did not show the similar results. In multivariable logistic analysis, the independent predictors of MACE were: MBG, MAGE and LVEF in diabetic subjects and were MBG and MAGE in nondiabetic subjects. Other factors were not significantly associated with MACE. Conclusions: Greater GV is associated with composite MACE and non-IRA revascularization during in-hospital and 30-day follow-up in unadjusted analyses, especially for diabetic subjects. After multivariable logistic analysis, GV remains an independent prognostic factor for composite MACE in STEMI patients undergoing p-PCI.