A firmware variable update method for BIOS of a computer system is provided. The firmware variable update method includes writing an update capsule file and a firmware update command into a memory according to a first system management interrupt signal and performing a shutdown process, triggering a second system management interrupt signal and accordingly writing a variable update command into the memory, switching the computer system into a sleep state before the shutdown process is completed and activating a timer, performing a first reboot process when the timer expires, utilizing the update capsule file to update an original capsule file of the BIOS according to the firmware update command during the first reboot process, and performing a second reboot process and utilizing an update variable file to update an original variable file of the BIOS according to the variable update command during the second reboot process.