The present invention relates to a state of charge (SOC) compensation method of a battery and a battery management system using the SOC compensation method. The charge and discharge current of the battery is used to calculate a first SOC and a first voltage corresponding to the first SOC, the charge and discharge current of the battery, the battery pack voltage, and an internal resistance of the battery are used to calculate a rheobasic voltage of the battery, an integration error corresponding to a difference between the first voltage and the rheobasic voltage is calculated, and a SOC compensation factor corresponding to the error is added to or subtracted from the first SOC to determine a more accurate SOC of the battery.