An abnormal connection detection method is used between a power supplier and a power receiver. The power supplier and the power receiver are connected through a cable. The cable includes positive and negative power transmission lines. The abnormal connection detection method includes: providing an output voltage from the power supplier, wherein the output voltage is lower than a predetermined voltage threshold; detecting, according to the output voltage, whether an output current generated by the power supplier is higher than a predetermined current threshold; and when the output current is higher than the predetermined current threshold, determining that an abnormal connection occurs between the power supplier and the power receiver.