Abstract: Acute myocardial infarction (AMI), a necrosis of the heart muscle caused by unstable ischemic syndrome, is the leading cause of death and disability in patients with coronary heart disease. For early diagnosis and treatment of AMI patients, it is necessary to combine multiple biomarkers and interpret the tissue or organ injury status and pathological mechanism of patients from multiple perspectives, establish personalized treatment plans, improve the long-term prognosis of patients and reduce the risk of major adverse cardiovascular events after AMI. miRNAs (micrornas) are a class of small molecule Rnas, which have been shown to be widely expressed and play an important role in many biological tissues, including in cardiovascular diseases, where mirnas are tissue and cell specific. They control the functions of various cells and the concentration of miRNA in cells can significantly affect the metabolism, cycle and function of cells, which may have potential value in the diagnosis and treatment of AMI. Among them, the correlation between plasma microRNA21 (miR-21) and AMI is mostly studied. Its important biological significance has been gradually confirmed and it is expected to become a new target for the diagnosis and treatment of AMI in the future. In this review, the research progress of miR-21 in the mechanism, diagnosis and treatment of AMI was reviewed and the future development direction was discussed.