A method and system are devised for quickly detecting an abnormal concentration of potassium ions in blood from an electrocardiogram. The system includes at least one vector-converting device that includes a processor, a transmitter, at least one memory and at least one storage unit. The at least one storage includes a model data module and a predicting and converting module. The model data module includes data of a model of the concentration of potassium in the blood. The model of the concentration of potassium in the blood includes at least one reference electrocardiogram and corresponding reference data of the concentration of potassium in the blood. The predicting and converting module converts an electrocardiogram into a corresponding predicted concentration of potassium in the blood according to the reference concentration of potassium in blood. The vector conversions device is connected to at least one electrocardiogram generator and at least one monitor. Thus, the electrocardiogram obtained by the electrocardiogram generator is converted to the corresponding predicted concentration of potassium in the blood. The corresponding predicted concentration of potassium in the blood is shown on a monitor to facilitate the medical personnel to take proper actions to reduce the risks of sudden cardiac death.