A universal serial bus (USB) testing method includes selecting a selected test mode from test modes, creating a USB communication link between a USB device and a testing fixture board, generating, by the testing fixture board, a test-triggering instruction corresponding to the selected test mode according to the selected test mode, sending the test-triggering instruction to the USB device with the USB communication link, generating, by the USB device, a testing packet corresponding to the selected test mode according to the test-triggering instruction, and outputting the testing packet repeatedly from at least one external port of the USB device.