Objective    To analyze different doses of metoprolol in prevention of atrial fibrillation (AF) after coronary artery bypass graft (CABG). Methods    From June 2016 to August 2017, 358 patients undergoing CABG in cardiothoracic surgery in Nanjing First Hospital were randomly divided into two groups according to the dose of metoprolol: a group A with metoprolol of 25 mg/d, a total of 182 patients, including 145 males and 37 females, with an average age of 65.40±10.52 years; a group B with metoprolol of 75 mg/d, a total of 176 patients, 138 males and 38 females with an average age of 63.31±9.04 years. The incidence of AF was observed 5 days after surgery. Results    The incidence of post-CABG AF (PCAF) in the group A and the group B was 27.47%, 18.18%, respectively with a statistical difference (P=0.04). PCAF was detected its maximum peak on the second day post-surgery. Of patients at age of 70 years or more, the incidence of PCAF in the group A was higher than that in the group B with no statistical difference (P=0.18). Among the patients with left ventricular ejection fraction (LVEF) lower than 40%, there was no statistical difference in the incidence of PCAF between the two groups (P=0.76). Conclusion    Metoprolol 75.00 mg/d is better than 25.00 mg/d in preventing new AF after CABG.