This study focused on miR-486-5p in atrial fibrillation (AF) evaluating its clinical significance and revealing its regulatory mechanism in cardiac fibroblasts, aiming to explore a novel biomarker for AF. The study enrolled 131 AF patients and 77 non-AF individuals. With the help of polymerase chain reaction (PCR), the expression of miR-486-5p was evaluated. The significance of miR-486-5p in the diagnosis of AF and the occurrence of left atrial fibrosis (LAF) was assessed by receiver operating curve (ROC) and logistic analyses. The regulatory effect and mechanism of miR-486-5p on cardiac fibrosis were investigated in human cardiac fibroblasts treated with angiotensin II. miR-486-5p was significantly upregulated in AF patients and discriminated AF patients from non-AF individuals. Increasing miR-486-5p showed a significant association with decreasing left ventricular ejection fraction (LVEF), increasing left atrial diameter (LAD) and left ventricular end-diastolic diameter (LVEDd), and the high incidence of LAF in AF patients. Moreover, miR-486-5p was identified as a risk factor for LAF and could distinguish AF patients with LAF and without LAF. In cardiac fibroblasts, angiotensin II induced the upregulation of miR-486-5p and promoted cell proliferation, migration, and collagen synthesis. miR-486-5p negatively regulated forkhead box O1 (FOXO1) and its knockdown could reverse the promoted effect of angiotensin II. FOXO1 alleviated the effect of miR-486-5p, and the miR-486-5p/FOXO1 could activate PI3K/Akt signaling. The activation of PI3K/Akt signaling alleviated the enhanced proliferation, migration, and collagen synthesis of cardiac fibroblasts induced by angiotensin II, and its inhibition showed opposite effects. Increased miR-486-5p served as a biomarker for the diagnosis and development prediction of AF. miR-486-5p regulated cardiac fibroblast viability and collagen synthesis via modulating the PI3K/Akt signaling through targeting FOXO1.