Abstract: A testing system includes a circuit board, and an inserting unit. The circuit board includes a first serial interface and a serial chip connected to the first serial interface. The first serial interface connects a second serial interface of a motherboard to receive a first signal of the second serial interface. The inserting unit includes a first plug connected to a pin Transmit Data of the first serial interface. The first plug connects a testing device. When the first signal is transmitted to the first serial interface by the second serial interface, the serial chip receives the first signal and sends the first signal back to the first serial interface. The first plug sends a second signal of the pin Transmit Data to the testing device to be tested.