Objective To investigate the effect of peripheral blood MDC on predicting adverse cardiac events of patients with acute STEMI. Methods Thirty-nine patients with acute ST segment elevation myocardial infarction admitted in the People′s Hospital of Deyang from February 2016 to August 2017 were selected by random sampling. According to 7-day adverse cardiac events, the patients were divided into non-combination adverse cardiac events group (uncomMI group, 23 patients) and combination adverse cardiac events group (comMI group, 16 patients), and 24 healthy persons were selected as normal control group. The general situation, clinical characteristics and MDC, PDC, analysis of uncomMI group and comMI group in the 1st and 3rd day after the onset of acute ST segment elevation myocardial infarction was compared. Factors influencing adverse cardiac events in patients with acute ST segment elevation myocardial infarction were analyzed. Results The peripheral blood MDC and plasmacytoid dendritic cell (PDC) counts in patients with STEMI were significantly lower than those in normal controls in early time(at the time of day 1)(P<0.05), but the peripheral blood MDC and PDC counts in uncomMI group was not significantly different from that in normal control group at the time of day 3 (P>0.05). The peripheral blood MDC and PDC counts in comMI group were lower than those in uncomMI group and control group. The Logistic analysis showed that the elderly patients, total cholesterol (TC), MDC (at the time of day 3) were all risk factors of 7-day adverse cardiac events (P < 0.05). The receiver operating characteristic (ROC) results showed that the maximum area under curve(AUC) was 0.8286 when the MDC truncation point was 7 mg/L, and the sensitivity was 80.0% and the specificity was 85.7% . Conclusions For STEMI patients, MDC of peripheral blood is associated with 7-day adverse cardiac events. The percentage of MDC in peripheral blood of patients with acute STEMI has a certain clinical value in the diagnosis of 7-day adverse cardiac events. In addition, elderly patients with STEMI have an increased risk of 7-day adverse cardiac events.