Objective: The study aimed to investigate whether a Shengmai San-derived herbal, Fumai granule (FM), which had a preventive effect on atrial fibrillation (AF) in myocardial infarction (MI)-induced heart failure (HF) rat and to determine the underlying mechanisms. Design and methods: MI was induced by a ligation of the left anterior descending coronary artery. One week after MI surgery, FM was gavaged for 4 weeks. AF inducibility was detected by transesophageal programmed electrical stimulation technology. Multielectrode array measurements, echocardiogram, histology, and western blotting were performed. Results: The FM-treated group had lower rates of AF inducibility and shorter AF duration compared to the MI group. FM improved the conduction velocity and homogeneity, decreased left atrial positive fibrosis areas and expression of type I and III collagen, inhibited cardiac fibroblast to myofibroblast differential, and increased the expression of connexin 43 and connexin 40 in the left atrium. Conclusions: These results suggest that FM reduced the AF inducibility after MI by improving the left atrial conduction function via inhibiting left atrial fibrosis and increasing the expression of connexin, indicating its benefit in preventing the MI-induced vulnerable substrate for AF.