A purge control method of an air tank may include compressing air in an air compressor, removing moisture from the air through a cartridge, storing the air in the air tank, measuring and determining an internal pressure of the air tank after storing the air in the air tank, operating a first solenoid valve if the internal pressure exceeds a first predetermined pressure, determining whether a second solenoid valve is operable and operating the second solenoid valve if operable, operating a purge valve to improve performance of the cartridge, thereby removing the moisture in the air tank, measuring and determining the internal pressure again, and turning off the first and second solenoid valves if the internal pressure is less than a second predetermined pressure.