Disclosed herein is a method of calculating the SOC of a battery. The method includes the steps of receiving information about a previous SOC mode and operation time in the mode from memory of a battery controller after startup of the vehicle; calculating a battery SOC data value using one of respective different reference capacities depending on whether a normal SOC preservation mode, a high SOC range expansion mode, or a low SOC range expansion mode using the battery controller and transmitting the battery SOC data value to a vehicle controller; and determining whether conditions for entering a next SOC mode is met after calculation of an actual SOC value of the battery in the normal SOC preservation mode, the high SOC range expansion mode, or the low SOC range expansion mode.