The emergency exit sign on civil aircraft is for the passengers and flight-crew to find the exit when the aircraft needs to take an emergency landing. At present, emergency exit signs on civil aircraft use single power supply mostly which limits the location of the emergency power supply and increases the maintenance cost. This article presents a new double power supply method to emergency exit sign and introduces the design theory in detail. The double power supply method can reduce the location limit of the emergency power supply and improve the airworthiness. The method can provide guidance for the design of emergency exit sign on civil aircraft.