This paper conducts a trade-off study on the two typical APU, mainly studying APU components, configuration, starting generator system, variable speed technology, fuel module and anti-surge control, to obtain the advantages and disadvantages of different body forms. Generally, the civil airliner APU adopts a structure of centrifugal compressor, reversed-flow type annular chamber and axial flow turbines. The APU bleeding air from the power compressor is small, light and high reliability, and is suitable for regional passenger planes that require small and light. The APU bleeding air from the load compressor has characteristics of low fuel consumption,good bleed air control, making it more suitable for large aircraft that requires low fuel consumption. Compared to the starter and generator separately, the APU integrated design starter/generator system had the advantages of light weight,longer service life, and higher reliability. The use of variable speed technology was more economical and more environmentally friendly, but it needed frequency conversion power and better control system to meet aircraft requirement. There was a risk of insufficient performance margin. Using a motor driven gear wheel pump, it was lightweight and had high control accuracy, but the control system was more complex and the fuel flow range was small. The APU required high fuel consumption, and the fuel system was involved in control guide vane and anti-surge, usually using a shaft driven pump. The APU bleeding air from the power compressor usually switches on and off the anti surge valve under specific working conditions. The APU bleeding air from the loaded compressor will be identified by compressor flow.