Objective To investigate the relationship between miR-3187-5p in peripheral blood and pericardial drainage after coronary artery bypass grafting (CABG) and postoperative atrial fibrillation (POAF). Methods Patients who underwent CABG in the Heart Center of Beijing Chao-Yang Hospital from March to May 2022 were enrolled. Peripheral blood and pericardial drainage were collected at 0 h after surgery (immediate time for patients to return to ICU from operating room) to detect miR-3187-5p, and perioperative confounding factors were also collected. The miR-3187-5p was measured by quantitative real-time PCR and its regulated target genes were analyzed by bioinformatics. Results A total of 15 patients were enrolled, including 9 males and 6 females with an average age of 65.6±8.2 years. The incidence rate of POAF was 40.0%. miR-3187-5p in pericardial drainage at 0 h after surgery was an independent predictor for POAF. A total of 1 642 target genes of miR-3187-5p were predicted. GO function enrichment analysis and KEGG signal pathway enrichment analysis showed that target genes of miR-3187-5p were enriched in TGF-β, MAPK, Wnt and other classical collagen metabolic signal pathways, which might activate collagen metabolism by negatively regulating SMAD6 and other inhibitors of the pathways. Conclusion This study is the first to find that miR-3187-5p in pericardial drainage at 0 h after surgery is a potential, novel, and predictive factor for POAF, which may be related to the regulation of myocardial fibrosis signal pathways like TGF-β, MAPK and Wnt pathways, promoting the early collagen metabolism imbalance after CABG, increasing the collagen deposition in the atrium, and then promoting the early structural reconstruction after CABG and leading to the occurrence of POAF. The result provides a research basis for the accurate prediction and prevention of clinical POAF.