The primary function of the auxiliary power unit system is to provide a standby electrical power while airplanes being in the air and on the ground, and to bleed air for engine start and environmental control system. As an important part of auxiliary power unit system, the start system is a guarantee of APU system to work normally. The electrical principle and electrical interface of APU start system on three typical configurations are introduced and analyzed. compared with all kinds of electricaldesign, the results show the development direction of the electrical design based on the APU start system for civil aircraft. The research can provide useful reference for APU startsystem design scheme in civil aircraft.