A signal relay system includes an input terminal, an output terminal, a signal detector, a signal repeater, an input terminal circuit, an output terminal circuit, a switch and a switch controller. The signal detector is for detecting a wakeup signal from the input terminal. The signal repeater is for receiving a transmission signal and to amplify and forward the transmission signal. The input terminal circuit is for reducing impedance between the input terminal and the signal repeater. The output terminal circuit is for reducing impedance between the output terminal and the signal repeater. The switch is for coupling the output terminal to the signal repeater or the input terminal. The switch controller is for operating the switch according to a detection result of the signal detector.