A testing circuit for testing a universal serial bus (USB) of an electronic device includes a controller, a first switch, a pull-down resistor, a gating pull-up resistor, and a second switch. The controller provides a control signal according to a power receiving condition of the electronic device. A control terminal of the first switch is coupled to the controller. The pull-down resistor is coupled between a configuration channel pin of the USB and a first terminal of the first switch. The gating pull-up resistor is coupled between the configuration channel pin and the control terminal of the first switch. A control terminal of the second switch is coupled to the controller. A first terminal of the second switch is coupled to a second terminal of the first switch and a ground pin of the USB. A second terminal of the second switch is coupled to a reference low voltage.