The invention discloses a power-on signal transmitting system, which includes a first electronic device, a second electronic device and a cable coupled between the two electronic devices. The first electronic device includes a power-on control unit. The second electronic device includes a power-on detection unit. The cable includes at least one information wiring and a signal level wiring. The at least one information wiring is used for transmitting an information signal between the first electronic device and the second electronic device. When the second electronic device is at a power-off state, the power-on control unit of the first electronic device is used for transmitting a power-on signal through the signal level wiring to the second electronic device. The power-on detection unit senses the power-on signal and turns on the second electronic device.