A PCI-Express bus is incorporated in a method for transmitting a power-saving command between a computer system and its plurality of peripheral devices of the present invention. More particularly, a specific power management command is introduced into the signal transmission protocol of a system command, which is transmitted between the plural system chips. Therefore, the peripheral devices coupled with the system chips can enter a certain power mode simultaneously. The present invention is used to solve the problem of the peripheral devices cannot enter the certain power mode since the system chip has no power management unit disposed under the PCI-Express structure.