Computer system and booting method thereof

A booting method is provided, applied to a computer system. The computer system includes a transmission interface, a power key, and a trigger element. The transmission interface includes a transmission specification and is electrically connected to a graphics card, and the power key is used for driving the computer system to perform a booting procedure. The booting method includes: detecting, in the booting procedure, whether the trigger element is triggered or not; and lowering the transmission specification and restarting the computer system when the trigger element is triggered. A computer system adopting the booting method is further provided.