The utility model discloses an automatic start-stop device for an external fan of a self-cooled transformer, and the device consists of a rack, a refrigerating fan, a temperature sensor, a temperature control switch, an alarm, a current and voltage detector, a rotating speed relay, and an emergency stop switch. The rack is disposed on a transformer housing. The refrigerating fan, the temperature control switch, the alarm, the current and voltage detector and the emergency stop switch are all disposed on the rack. The temperature sensor is disposed on the transformer housing, and is electrically connected with the temperature control switch. The current and voltage detector is electrically connected with a binding post of a transformer. The rotating speed relay is disposed on a rotating shaft of the refrigerating fan. The device provided by the utility model is high in degree of automation, is high in operation precision, is stable and reliable in operation of a system, can effectively decrease the external temperature of the transformer, effectively reduces the energy consumption of the operation of the fan, has the functions of cooling the fan, monitoring operating status of the fan and giving an alarm, and prevent the temperature decrease of the transformer from failing caused by insufficient power or a fault.