The invention discloses an automatic power-on or power-off modulator-demodulator device, comprising a power supply module, a power-on/power-off control module, a control module, a signal detection module, a modulator-demodulator network port, a modulator-demodulator module and a computer network port. The device is characterized in that the power supply module is connected with the power-on/power-off control module, the control module and the signal detection module; the control module controls the power-on/power-off control module and the signal detection module; the power-on/power-off control module is connected with the modulator-demodulator module; the signal detection module is connected with the modulator-demodulator network port; and the modulator-demodulator network port is connected with the computer network port. According to the modulator-demodulator device provided by the invention, the modulator-demodulator network port is monitored through the signal detection module; and when the computer is powered on, the modulator-demodulator device is powered on, when the computer is powered off, the modulator-demodulator device is powered off, the use time of the modulator-demodulator device is shortened, and moreover, electric energy is saved.