Aim Detection of relative expression levels of plasma miR-21 by a case-control study of acute aortic dissection (AAD), combined with receiver operating curve (ROC) and related clinical data to evaluate the diagnostic value of miR-21 for AAD. Methods 30 samples of AAD patients, 30 samples of angina patients, 29 patients with acute myocardial infarction (AMI) and 29 patients in the control group, clinical data of each group were also collected. The plasma levels of miR-21 and housekeeping gene miR-16 in each group were identified by quantitative reverse transcriptase polymerase amplification reaction, and the relative expression of miR-21 was calculated. Results Compared with the control group, the relative expression level of miR-21 in plasma of AAD group were significantly increased (P<0.05). There was no significant difference in plasma miR-21 expression between angina pectoralis group vs AAD group and AMI group vs AAD group (P>0.05). ROC curve analysis showed that the area under the curve (AUC) was 0.664.The 95% confidence interval was 0.520~0.809, and the sensitivity of plasma miR-21 to AAD diagnosis was 40%, and the specificity was 96.6%. When the comparative expression of miR-21 was 0.034, the sensitivity of plasma miR-21 to AAD diagnosis was 80%, and the specificity was 20.7%. Conclusion The detection of plasma miR-21 has definite clinical value for the diagnosis of AAD, but it is not enough to support miR-21 as an ideal molecular diagnostic marker for AAD.