Objective:Discuss ion magnesiums value in extracorporeal circulation.Methods:40 patients with rheumatic heart disease were enrolled to detect the content of ion magnesium at different stage in ECC,heart automatic beat rate and ventricular arrhythmia rate to find out the ion magnesiums value.Results:The concentration of ion magnesium will decline obviously during ECC,and have distinct relation of heart automatic beat and ventricular arrhythmia.Conclusion:Properly management of ion magnesium in ECC can improve heart automatic beat rate evidently and reduce obviously the rate of ventricular arrhythmia.