Objective: To explore a new method of differential diagnosis of fetal double aortic aorta (DAA) and right aortic arch with mirror-image branches (RAA-MB). Methods: Clinical data and prenatal echocardiographic features of the DAA (n = 22) and RAA-MB (n = 65) confirmed by postnatal or autopsy findings were analyzed retrospectively. The angles between the two aortic arches in the DAA group and between the right aortic arch and the mirror branch were measured. The differences between both groups and differential diagnosis value of the angles were compared and analyzed based on the ROC curve. Results: Left-sided DAA’s proportion (100%) was higher in the double aortic arch group (32.3%) than in the RAA-MB group, (P ＜ 0.05). The proportion of conotruncal anomalies is higher in the RAA-MB group (64.6%) than in the DAA group (18.2%) (P ＜ 0.05). There was a significant difference in the angles between both groups (DAA: 50.3° ± 8.3° vs RAA-MB: 82.9° ± 13.8°) (P < 0.01). When the cut-off value was 62.8°, the sensitivity and specificity of differential diagnosis were 95.5% and 96.9%, respectively. Conclusions: Distinguishing the angle measurement between DAA and RAA-MB is helpful in prenatal prognosis. We recommend a cutoff value of 62.8°.