An over-temperature protection system has a control board mounted in a charging device and a temperature sensor mounted in a charging plug. A switch is coupled on a power circuit of the charging plug and controlled by the control board. When the charging plug is connected to a power supply and the charging device begins a charging operation, the control board periodically receives temperature information of the charging plug sensed by the temperature sensor. When the control board determines that the temperature of the charging plug is abnormal, the switch is open to interrupt the power circuit so as to stop the charging operation for ensuring safety of the charging device.