Background Atrial fibrillation (AF) is a common complication of acute myocardial infarction (AMI), significantly affecting patient prognosis by increasing the risk of heart failure, thromboembolic events, and mortality.Objective To explore the risk factors of AM combined with first-diagnosed AF.Methods Clinical data related to a total of 142 consecutive AMI patients admitted to Beijing Tongren Hospital from January 1, 2020 to May 1, 2020 were retrospectively analyzed, of whom 8 cases (5.6%) were divided into first-diagnosed AF group and the rest 134 cases were in non-first-diagnosed AF group according to the presence or absence of first-diagnosed AF. Clinical data, laboratory indexes, cardiac ultrasonography and coronary angiography results of the two groups were compared. Logistic regression method was used to analyze the possible risk factors offirst-diagnosed AF.Results Comparing the various data between the two groups, the peak creatinine kinase myocardial band (CKMB), white blood cell count, left ventricular ejection fraction, and the proportion of patients with admission Killip grades 3-4 in the first-diagnosed AF group were significantly different from those in the non- first-diagnosed AF group (p < 0.05); the results of logistic multifactorial regression analysis showed that admission Killip grades 3-4 was an independent risk factor for first-diagnosed AF in patients with AMI (OR = 9.549, 95% CI: 1.697-53.737, p = 0.01).Conclusion Patients with AMI and Killip grade 3-4 face a heightened risk of developing atrial fibrillation. Close clinical observation, including ECG monitoring, is essential for early atrial fibrillation detection. Given the limitations of the current study, such as small sample size in single center, small number of AF events, the retrospective design, the validity and reliability of the research findings are constrained.