Globally acute myocardial infarction is the leading independent cause of unexpected death. This study aimed to explore the diagnostic and molecular impact of miR-21, miR-488, and miR-126 in acute myocardial infarction patients (AMI). We enrolled 95 non-ST-elevation myocardial infarction (NSTEMI) patients, 152 ST-elevation myocardial infarction (STEMI) patients, and 95 healthy individuals, additionally using three-month-old mice and their ventricular-derived H9c2 cells. The circulatory plasma miR-21 and miR-488 levels were significantly upregulated, while plasma miR-126 levels were remarkably downregulated in NSTEMI and STEMI subjects. The receiver operating characteristic curve showed that plasma miR-21, miR-488, and miR-126 were able to clearly differentiate NSTEMI and STEMI from healthy subjects. Moreover, H9c2 hypoxic cells treated with inhibitor miR-21 markedly reduced intracellular ROS levels, capase-3 activities levels, and cellular apoptosis rates and significantly enhanced cellular viability through up regulation of Smad-7 mRNA and protein expressions. In geriatric STEMI and NSTEMI subjects, plasma miR-21 levels were evidently higher than in comparatively younger subjects. Upregulated plasma miR-21 and miR-488 levels and downregulated miR-126 levels might be considered potential clinical biomarkers for myocardial infarction patients, while inhibition of miR-21, which significantly reduced hypoxia-exposed H9c2 cellular injury via targeting Smad-7, could be a new therapeutic target for AMI patients. Low levels plasma miR-21 may have a significant impact on delaying the aging process.