Atrial fibrillation (AF) is a commonly encountered heart arrhythmia and a risk factor for cardiovascular system. The purpose of the present study was to explore the role of long non-coding RNA myocardial infarction-associated transcript (MIAT) in AF and AF-induced myocardial fibrosis and the possible mechanisms involved in this process. We successfully induced an AF rat model. Expression of MIAT presented a dramatic increase, while microRNA (miR)-133a-3p presented a dramatic decrease in atrium tissues of rats with AF induction. In addition, we also found that MIAT was highly expressed and miR-133a-3p was significantly reduced in peripheral blood leukocyte of AF patients. For biological function exploration of MIAT/miR-133a-3p axis, MIAT was knocked down using small hairpin RNA (shRNA) lentivirus injection and the rescue experiments were performed simultaneously by inhibiting miR-133a-3p using anti-miR-133a-3p lentivirus injection in rats with AF. MIAT downregulation significantly alleviated AF, increased atrial effective refractory period (AERP), and reduced the duration of AF as well as cardiomyocytes apoptosis. Whereas these effects of MIAT downregulation on AF were reversed by anti-miR-133a-3p administration. Luciferase reporter revealed that miR-133a-3p was directly regulated by MIAT. Moreover, MIAT knockdown effectively reduced AF-induced atrial fibrosis by detecting reduced collagen in the right atria and inhibited expression of fibrosis-related gene expression of collagen I, collagen III, connective tissue growth factor (CTGF) and transforming growth factor-β1 (TGF-β1) in rats with AF, these findings were in contrast with the findings for rats with inhibition of miR-133a-3p. In conclusion, our study demonstrated the role of MIAT downregulation in alleviating AF and AF-induced myocardial fibrosis, and the functional regulatory pathway of MIAT targeting miR-133a-3p.