Auxiliary Power Unit (APU) system in-flight starting design and certification involve APU development, APU system inlet and exhaust ram recovery analysis, APU inlet/ exhaust impact on APU performance analysis, APU inlet door design, inlet door aerodynamic load analysis, inlet door actuation mechanism design, inlet door control logic design, APU starting control logic design, ram recovery flight test, certification flight test. APU system in-flight starting design has an impact on APU performance and in-flight starting envelope. This paper presents a design of Auxiliary Power Unit system in-flight starting. Based on the development and research experience,it summarizes the APU system design and certification process and method. It is a good reference to other aircraft model development.