A portable computer having a display part and a main part includes a magnetic piece installed in the display part, a Hall sensor installed in the main part, a switch circuit, and a control terminal connected to a power supply of a back light module of the display part. The Hall sensor includes a power pin connected to a first DC power supply, an output pin, and a ground pin connected to ground. The output pin of the Hall sensor is connected to the control terminal and a second DC power supply via the switch circuit, when the display part is in a closed position, the Hall sensor senses the magnetic field of the magnetic piece and outputs a control signal to the switch circuit, the switch circuit receives the control signal and turns off the power supply of the back light module via the control terminal.