The present invention relates to an automatic transformer protection circuit and an electric vehicle electricity supplying system using the same. The present invention includes an automatic transformer; an instrument transformer which is connected to the power port of the automatic transformer and detects the current of an electric vehicle line, a rail, and an electricity supplying line; a current ratio detector which is connected to the instrument transformer and operates when a difference current occurs after the difference current values of the electric vehicle line, the rail, and the electricity supplying line are compared; an opening/closing unit which is connected to the electric vehicle line and the electricity supplying line and separates the automatic transformer from the main line by being linked with the current ratio detector; a control unit which is connected to the automatic transformer, the current ration detector, and the opening/closing unit, operates the opening/closing unit when a difference current occurs, and generates an alarm; and a top/bottom line tying/blocking unit which includes a method to separate the automatic transformer from the main line automatically or manually during a load (pressurization) state and a no-load (non-pressurization) state and ties or blocks the top or the bottom line in case one automatic transformer among SSP-type automatic transformers installed at the top or the bottom line is separated due to a failure or an inspection. [Reference numerals] (11) System board;(3) Automatic transformer;(AA) Communication;(BB) TF electric rail car line;(CC) AF electricity supply line