Abstract: A test system for testing a network device is provided. The test system includes a control device and a cable group. The control device receives a user instruction, generates a test instruction according to the user instruction, and includes a control board and a relay board. The control board parses the test instruction, and generates a control signal. The relay board is connected to the control board, and comprises a plurality of switches, various of the switches are switched on or off according to the control signal. The cable group includes a plurality of cables with different lengths, the cables electrically connected to the switches, for forming a test line with a required length according to the control signal. A system and a method for automatically adjusting a length of a test line are also provided.