A serial data transmission system includes a sending terminal for sending data, a receiving terminal for receiving the data sent by the sending terminal, a first connecting capacitor connected between the sending terminal and the receiving terminal, and a second connected capacitor connected between the sending terminal and the receiving terminal. The sending terminal includes a sending terminal driving unit, and an amplitude detecting unit connected to the sending terminal driving unit. The sending terminal driving unit outputs a pair of differential signals according to signals of the received data. The amplitude detecting unit detects changes in amplitudes of the differential signals outputted by the sending terminal driving unit, and outputs an indicating signal for indicating whether the sending terminal is properly connected to the receiving terminal. A serial data transmission method is further provided.