When a controller identifies a universal serial bus (USB) device connected to a USB interface, it outputs a control signal to close a relay and a first start test signal to an electronic device to test a USB interface of the electronic device. When the test of the USB interface is completed, to the controller adds one to an inside counter, and determines whether the count value reaches a preset count value. If the count value reaches the preset count value, the controller outputs a finish test signal to the electronic device. If the count value does not reach the preset count value, the controller controls the closed relay to open and outputs a second start test signal to close a next relay to test a next USB interface of the electronic device corresponding to the now closed relay.