Inclusion Criteria:  Age ≥18 years old, <85 years old, no gender limit; Postoperative atrial fibrillation in the ICU after cardiac surgery; The duration of atrial fibrillation> 1 minute, and ≤ 48 hours; Hemodynamically stable (no need to increase vasoactive drugs and SBP>90/MAP>60mmHg); After pre-treatment (including: correcting electrolyte disturbances, optimizing volume status, improving oxygenation, controlling body temperature, analgesia and minimizing the use of inotropes and vasopressors), the clinician believes that antiarrhythmic drugs are needed. Obtained the informed consent from the patients or their family members.  Exclusion Criteria:  Heart transplantation, left heart assist device (LVAD) or extracorporeal membrane oxygenation (ECMO) treatment; History of atrial fibrillation/atrial flutter and a history of paroxysmal supraventricular tachycardia; Radiofrequency ablation; Rheumatic heart disease; Complex congenital heart disease (with more than two coexisting congenital heart defects); Cardiac tumors; Transcatheter aortic valve implantation (TAVI), transcatheter mitral valve intervention (TMVI), and transcatheter tricuspid valve intervention (TTVI); Contraindications to amiodarone/nifekalant (PR interval>240ms; 2nd or 3rd degree atrioventricular block (AVB); QT>440ms; familial long QT syndrome; Untreated thyroid disease; AST or ALT>2 times the upper limit; liver cirrhosis; interstitial lung disease); Heart rate (HR) <50 beats/min and/or QRS>140ms without a pacemaker; Received amiodarone or nifekalant within 6 weeks before the operation; Pregnant and lactating female patients; Uncorrected hypokalemia (serum potassium <3.5mmol/L) or hypomagnesemia (whole blood/serum magnesium below the lower limit); Chronic renal failure and/or continuous renal replacement therapy (CRRT); Return to OR during ICU stay or readmission to ICU from Cardiac Surgery ward. Other factors not suitable for participating in this study