An electronic display system and a screen brightness adjusting method thereof are disclosed. The electronic display system includes an electronic display device and a signal connection cable. The electronic display device is electrically connected to a computer system via the signal connection cable, and the signal connection cable can be selectively connected to a mains power supply. The electronic display device comprises a connection port, a control chip, a screen, and a screen brightness adjustment module. After connecting the signal connection cable, the control chip determines whether the signal connection cable is connected to the power supply end to cause the screen brightness adjustment module controls an adjustable maximum brightness of the screen to be equal to a maximum default brightness or lower than the maximum default brightness.